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SUMMARY TECHNICAL REPORT 


OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


DECLASSIFIED 
By authority Secretary of 

OCT 101960 

n o August i960 
Defense memo^A * 

OF CONGRESS 


Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the Colum¬ 
bia University Division of War Research under contract 
OEMsr-1131 with the Office of Scientific Research and De¬ 
velopment. This volume was printed and bound by the 
Columbia University Press. 

Distribution of the Summary Technical Report of NDRC 
has been made by the War and Navy Departments. Inquiries 
concerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other refer¬ 
ence material should be addressed to the War Department 
Library, Room 1A-522, The Pentagon, Washington 25, D. C., 
or to the Office of Naval Research, Navy Department, Atten¬ 
tion : Reports and Documents Section, Washington 25, D. C. 

Copy No. 

189 


This volume, like the seventy others of the Summary Tech¬ 
nical Report of NDRC has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may 
be errors of fact not known at time of printing. The author 
has not been able to follow through his writing to the final 
page proof. 


Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D.C. 

A master errata sheet will be compiled from these reports 
and sent to recipients of the volume. Your help will make 
this book more useful to other readers and will be of great 
value in preparing any revisions. 


SUMMARY TECHNICAL REPORT OF DIVISION 17, NDRC 


VOLUME 3 


TRANSMISSION AND RECEPTION 
OF SOUNDS UNDER COMBAT 
CONDITIONS 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VAXNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B . C O N A N T , CHAIRMAN 

DIVISION 17 

GEORGE R. HARRISON, CHIEF 


DECLASSIF IED 
By authority Secretary of 

OCT 10 1960 

Defense memo 2 August 1960 
T.TPRARY OF CONGRESS 


WASHINGTON, D. C., 1946 




NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative- 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 

2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 
Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 
Commodore H. A. Schade 

3 Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Ooms 


1 Army representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 

Col. E. A. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suit¬ 
able projects and research programs on the instrumen¬ 
talities of warfare, together with contract facilities for 
carrying out these projects and programs, and (2) to 
administer the technical and scientific work of the con¬ 
tracts. More specifically, NDRC functioned by initiating 
research projects on requests from the Army or the 
Navy, or on requests from an allied government trans¬ 
mitted through the Liaison Offiee of OSRD, or on its 
own considered initiative as a result of the experience 
of its members. Proposals prepared by the Division, 
Panel, or Committee for research contracts for perform¬ 
ance of the work involved in such projects were first re¬ 
viewed by NDRC, and if approved, recommended to the 
Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of 
the contract, including such matters as materials, clear¬ 
ances, vouchers, patents, priorities, legal matters, and 
administration of patent matters were handled by the 
Executive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 

These were: 

Division A—Armor and Ordnance 
Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communication and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three 
administrative divisions, panels, or committees were 
created, each with a chief selected on the basis of his 
outstanding work in the particular field. The NDRC 
members then became a reviewing and advisory group 
to the Director of OSRD. The final organization w^as 
as follows: 

Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 



DECLASSIFIED 

NDRC FOREWORD By authority Secretary of 


A S events of the years preceding 1940 re- 
l vealed more and more clearly the serious¬ 
ness of the world situation, many scientists in 
this country came to realize the need of organ¬ 
izing scientific research for service in a national 
emergency. Recommendations which they made 
to the White House were given careful and 
sympathetic attention, and as a result the Na¬ 
tional Defense Research Committee [NDRC] 
was formed by Executive Order of the Presi¬ 
dent in the summer of 1940. The members of 
NDRC, appointed by the President, were in¬ 
structed to supplement the work of the Army 
and the Navy in the development of the in¬ 
strumentalities of war. A year later, upon the 
establishment of the Office of Scientific Research 
and Development [OSRD], NDRC became one 
of its units. 

The Summary Technical Report of NDRC is 
a conscientious effort on the part of NDRC to 
summarize and evaluate its work and to present 
it in a useful and permanent form. It comprises 
some seventy volumes broken into groups cor¬ 
responding to the NDRC Divisions, Panels, and 
Committees. 

The Summary Technical Report of each Divi¬ 
sion, Panel, or Committee is an integral survey 
of the work of that group. The first volume of 
each group’s report contains a summary of the 
report, stating the problems presented and the 
philosophy of attacking them, and summarizing 
the results of the research, development, and 
training activities undertaken. Some volumes 
may be “state of the art” treatises covering 
subjects to which various research groups have 
contributed information. Others may contain 
descriptions of devices developed in the labora¬ 
tories. A master index of all these divisional, 
panel, and committee reports which together 
constitute the Summary Technical Report of 
NDRC is contained in a separate volume, which 
also includes the index of a microfilm record 
of pertinent technical laboratory reports and 
reference material. 

Some of the NDRC-sponsored researches 
which had been declassified by the end of 1945 
were of sufficient popular interest that it was 
found desirable to report them in the form of 
monographs, such as the series on radar by 
Division 14 and the monograph on sampling 
inspection by the Applied Mathematics Panel. 
Since the material treated in them is not dupli¬ 
cated in the Summary Technical Report of 
NDRC, the monographs are an important part 


of the story of these »sperts| o& NftjfaC research. 

In contrast to the information on radar, 
which is of widespread interest and much of 
which is releai^Sfi^T^^l&.^HgH^i^Mi on 
subsurface warfare is_ln.rgely is 

of general inteHSt^ttHtTnore res trie tea group. 
As a consequence, the report of Division 6 is 
found almost entirely in its Summary Technical 
Report, which runs to over twenty volumes. 
The extent of the work of a division cannot 
therefore be judged solely by the number of 
volumes devoted to it in the Summary Tech¬ 
nical Report of NDRC: account must be taken 
of the monographs and available reports pub¬ 
lished elsewhere. 

The research work of Division 17 included a 
wide variety of projects, ranging from the de¬ 
tection of land mines to the characteristics of 
the human ear, from helium purity indica¬ 
tors to the telemetering of strain gauges, from 
odographs to sound-ranging devices. It is a 
tribute to the broad knowledge of the Division 
Chiefs—Paul Klopsteg and, later, George R. 
Harrison—and to the versatility of the men 
who worked under them that so diverse a pro¬ 
gram was handled so competently. 

A considerable portion of the work of Divi¬ 
sion 17 had to do with the shattering noise of 
modern war, and answers were sought and sup¬ 
plied to such questions as: How much noise can 
a human being stand? What clues must the 
human ear have in order to understand a 
spoken message? How much distortion can be 
tolerated? These and other phases of the Divi¬ 
sion’s work are dealt with in the Summary 
Technical Report prepared under the direction 
of the Division Chief and authorized by him 
for publication. 

The diversity of the Division’s projects made 
it inevitable that its staff should be composed 
of men with many types of scientific training 
and that the Division should draw on contrac¬ 
tors with a wide range of experience and skills. 
The studies of noise, in particular, meant that 
the technical staff must include physicists, 
acousticians, and psychologists. For the ability 
and devotion of these men of many aptitudes 
we express our gratitude. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. CONANT, Chairman 
National Defense Research Committee 





FOREWORD 


T he applied physics division of the Office 
of Scientific Research and Development 
[OSRD] was organized late in 1942 under the 
chairmanship of Dr. Paul E. Klopsteg, who was 
responsible for the work of the Division until 
shortly before the completion of its work when 
other duties required his full attention. Most 
of the projects which had been initiated by the 
Instruments Section of the National Defense 
Research Committee [NDRC] during 1940 and 
1941 and which were not concerned with optics 
were turned over to the Applied Physics Divi¬ 
sion on its inauguration. Dr. Klopsteg as Chief 
and Dr. E. A. Eckhardt as Deputy Chief went 
with them from the Instruments Section to the 
new Division. 

The Summary Technical Report which is pre¬ 
sented in these volumes thus covers the accom¬ 
plishments of projects set up by both Section 
D-3 and Division 17. The work of the Division 
covered a very wide range of fields. The term 
Applied Physics served in lieu of a more de¬ 
scriptive name for a Division which was in fact 
the one to which was assigned any scientific 
problem which did not properly come under one 


of the other Divisions of NDRC. 

Actually the Division was an association of 
three Sections having rather dissimilar respon¬ 
sibilities and fields of activity. In setting up 
these Sections it was necessary to group the 
projects already under way into a small num¬ 
ber of coherent categories, and those chosen 
were Sound, Electricity, and General Instru¬ 
mentation. The work of the Division consisted 
entirely of the integrated efforts of these three 
Sections, whose membership will be found listed 
on a succeeding page. 

For more detailed reports on the technical 
work of the Division than are contained here¬ 
with the detailed contractors’ reports of Divi¬ 
sion 17 should be consulted, and appropriate 
reference to these have been made throughout 
the present volumes. The results obtained are 
also presented in less technical form in that 
volume of the history of OSRD entitled Optics 
and Applied Physics in World War II. 


George R. Harrison 
Chief, Division 17 


DECLASSI FIED 
By authority Secretary of 

OCT 10 I960 

Defense me mo 2 A ugust i960 
JJBRARY OF CONGRESS 


vii 











PREFACE 


The research and development program of 
Division 17, NDRC, was concerned with those 
problems in physics not specifically covered in 
other Divisions of NDRC. As the result, the 
Division fell heir to a myriad of miscellaneous 
problems of a physical nature which, in them¬ 
selves, were not often interrelated. It would 
have been exceedingly difficult, if not impossible, 
for Division 17 to set up within itself a sufficient 
number of Sections to deal specifically with all 
the various classes of problems which fell under 
their jurisdiction. Therefore, the projects of the 
Division were assigned to one of three Sections 
—Section 17.1, Instruments; Section 17.2, Elec¬ 
trical Equipment; and Section 17.3, Acoustics— 
whose broad titles permitted a general, even if 
somewhat loose classification. It was not always 
easy to decide under which of these three broad 
categories a given project should be placed. In 
these cases, considerations such as immediate 
convenience and availability of experienced 
personnel were often the determining factors. 

The Summary Technical Report describing 
the activities of Division 17 is presented in four 
volumes. In an attempt to achieve a little 
greater uniformity of subject matter, the 
projects were organized within the various 
volumes without regard to their Section classi¬ 
fication. Consequently, there is, on the whole, 
little relationship between volume and Section 
number. Because of the varied problems dealt 
with in the Division’s program, very little con¬ 
tinuity is to be found from chapter to chapter 
in any volume. Each chapter attempts to sum¬ 
marize independently the results of a particular 
project. 

Since there were a large number of diversified 
projects in Division 17, it was impossible to do 
justice to each, even in summary. It is not in¬ 
tended that the importance of any project 
described herein should be judged by the 
amount of page space allotted to it. Naturally, 
certain problems involved more research and 
development than others before they could be 
brought to a successful conclusion. In many 
cases, this is reflected in the Summary Technical 
Report. On the other hand, the presentation of 


the projects may mirror the amount of enthu¬ 
siasm of the individual author at the time of 
writing. Therefore, the reader who desires more 
than a broad panorama of the Division’s activ¬ 
ities is referred to the Microfilm Index for 
more complete details. 

Volume 3 of the Division 17 Summary Tech¬ 
nical Report deals with the Transmission and 
Reception of Sounds under Combat Conditions. 
This entire program was carried out at the 
Psycho-Acoustic and Electro-Acoustic Labora¬ 
tories of Harvard University. As a result, the 
research on this program was more closely 
integrated and coordinated than many of the 
other Division 17 programs which were often 
divided among a number of different con¬ 
tractors. It was only natural, then, that the 
report summarizing the results of the investiga¬ 
tions of the two Harvard Laboratories should 
be a single volume. This is especially true since 
the chapters bear a logical relation to each 
other and present a fairly complete picture of 
the problems involved in the transmission and 
reception of sounds under conditions of com¬ 
bat. There were many other projects in Division 
17 on the transmission, reception, and simula¬ 
tion of sound, but they have been reported in 
Part II of Volume 2 rather than here. These 
other projects were mainly concerned with the 
simulation of combat noises and sonic decep¬ 
tion; hence, the problems of transmission and 
reception, while important, were more of a 
secondary nature than those dealt with in this 
volume, where they are of fundamental im¬ 
portance. 

All the chapters of this volume, except the 
Introduction, were written by G. A. Miller and 
F. M. Wiener of the Harvard Psycho-Acoustic 
Laboratory under the guidance of S. S. Stevens, 
its Director. These men merit high praise for 
their conscientious efforts in preparing their 
manuscript, and to them the Editor of this 
volume wishes to express his deep appreciation. 


Chas. E. Waring 
Editor 


IX 




CONTENTS 


CHAPTER PAGE 

1 Introduction by S. S. Stevens . 1 

2 Sound Control by F. M. Wiener and G. A. Miller .... 4 

3 Some Characteristics of the Human Ear by G. A. Miller and 

F. M. Wiener .47 

4 Some Characteristics of Human Speech by F. M. Wiener and 

G. A. Miller .58 

5 Articulation Testing Methods by G. A. Miller .69 

6 Intelligibility of Speech: Special Vocabularies by G. A. Miller 81 

7 Intelligibility of Speech: Effects of Distortion by G. A. Miller 86 

8 Intelligibility of Speech: Types of Interference by G. A. Miller 109' 

9 The Interphone by F. M. Wiener and G. A. Miller . . . 119 

10 Test Methods and Equipment for Interphone Components by 

F. M. Wiener .142 

11 The Design and Development of Certain Interphone Com¬ 
ponents by F. M. Wiener and G. A. Miller .174 

12 Special Voice Communication Systems by F. M. Wiener . . 188 

13 Noise Reduction in Radio Receivers by G. A. Miller . . . 197 

14 Selecting and Training Personnel by G. A. Miller . . . . 208 

15 Hearing Aids by F. M. Wiener and G. A. Miller .... 216 

16 Sonic Positioning Devices and Direction Finding by F. M. 

Wiener .233 

17 Sonic True Airspeed Indicator by F. M. Wiener .... 240 

18 Auditory Signals for Instrument Flying: “Flybar” by G. A. 

Miller .255 

19 Special Devices for Use on Shipboard by F. M. Wiener . 261 

Glossary.271 

Bibliography.275 

OSRD Appointees.284 

Contract Numbers.285 

Service Project Numbers.289 

Index.291 












































































Chapter 1 

INTRODUCTION 


11 THE SCIENCE OF SOUND IN 
WARTIME 

S ound is an important part of man’s wartime 
environment. Sound waves in air, water, 
and other media are important and sometimes 
useful as physical phenomena, but mostly they 
are significant because men have ears to hear 
them. On waves of sound men “pass the word,” 
a process which forms the backbone of effective 
military coordination. 

Troops, tanks, ships, planes are welded into 
effective working units by means of speech, 
conveyed either directly through the air or over 
radios, interphones, or telephones. But modern 
warfare is noisy. To the cracking of rifle fire 
and the blast of artillery has been added the 
more sustained din of innumerable mechanized 
vehicles and devices. The problems of reducing 
noise, protecting personnel, avoiding detection 
by the enemy or confusing his intelligence are 
issues that must be faced by a modern armed 
force. 

Back in 1940, when national defense was 
becoming the key to the national consciousness, 
there was mounting evidence that the science 
and art of military acoustics was in many par¬ 
ticulars inadequate to the threatened demands. 
It was becoming increasingly clear that much 
of our communication equipment had too little 
power, too much distortion, and too great a 
vulnerability to noise. Tanks and airplanes 
were becoming noisier and soldiers were being 
threatened with deafness by din and blast. Spe¬ 
cial acoustic problems and applications were 
coming to the fore: direction finding by means 
of sound waves, acoustic warning and signaling 
systems, sonic airspeed indicators, etc. 

To this state of affairs, the answer, of course, 
was research—scientific inquiry into the con¬ 
ditions of use, the causes of failure, the speci¬ 
fications for success. Take, for example, the 
problem of noise. Guns and motors presented 
hazards to voice communication seldom consid¬ 
ered in peacetime laboratories. What does the 
terrific noise of modern warfare do to men? 


What levels of noise can be tolerated? How can 
excessive noise be reduced and controlled ? How 
loud does speech have to be for men to hear it 
above the racket? When the problem of noise 
was taken into the laboratory, scientists deter¬ 
mined the characteristics of typical noises, 
simulated them, and studied their effects. Often 
this required the development of new techniques 
or the revision of old ones. Physicists and psy¬ 
chologists worked side by side for five years on 
these problems, and the job was still not com¬ 
plete when the war ended. 

It was quickly recognized that the human 
factor in communication is crucial. What clues 
must the human ear have to hear a spoken 
message, and what is the nature of the human 
voice itself? How much distortion can be toler¬ 
ated by the listener ? How loud must the speech 
be to stand out above the noise? And even when 
the voice and ear were accounted for, there was 
still the very human demand for comfort and 
convenience. Earphone sockets must exclude 
noise, but they must not crush the listener’s 
ears. Microphones must be held against the lips, 
but they should not occupy the talker’s hands. 
The human factor again appeared when it was 
necessary to select and train communication 
personnel. And even when the war was over 
the human problems of communication were 
not at an end, for deafened veterans needed 
hearing aids and general rehabilitation. At 
every point the experimental psychologist was 
called upon to effect an harmonious union of 
man and machine. 

The recognition of the human factor would 
have been of little value, however, if trained 
physicists and acousticians had not been avail¬ 
able to implement the psychologist’s recom¬ 
mendations with adequate equipment. This 
work did not end with the evaluation of exist¬ 
ing or proposed equipment, but on many occa¬ 
sions required the engineering of a new device. 
Even when the device had been accepted, it was 
often necessary to assist manufacturers in 
setting up testing methods and production short 
cuts. In addition to development, more difficult 


1 


2 


INTRODUCTION 


problems of measurement had to be solved. 
When airplanes began reaching altitudes of 
40,000 ft, communication failed. Reduced pres¬ 
sure lowered the sensitivity of microphones 
and earphones, changed the nature of the 
spoken word. It was necessary to devise meth¬ 
ods of measuring performance at high altitudes 
and at the same time to make practical sug¬ 
gestions for improvements. 

These are some of the problems discussed in 
detail in the following 18 chapters. These chap¬ 
ters have been organized around specific prob¬ 
lems, rather than around specific contracts or 
chronological sequence. It is hoped that these 
chapters will form an introduction to the prob¬ 
lem of reliable military voice communication 
and to the present state of our knowledge about 
it. Fundamental information is occasionally in¬ 
troduced in order to make the story more com¬ 
plete, and the physical and psychological aspects 
are treated together in summary as they were 
studied together in war. Since the organization 
and administration of the research is largely 
ignored in the following chapters, these aspects 
are briefly summarized in the following section. 


1.2 the three laboratories 

Unlike most Office of Scientific Research and 
Development [OSRD] projects, which fitted 
relatively neatly into one or another of the 
standard scientific fields, the research program 
in communications and airborne acoustics at 
Harvard was peculiarly an interdisciplinary 
venture. Problems related to sound and com¬ 
munications arise in almost every wing of the 
military structure: in airplanes, on shipboard, 
in tanks, in submarines, in diving suits, in pill¬ 
boxes, and in the rehabilitation clinics that 
follow as an aftermath of war. Furthermore, 
since both gadgets and men are involved in 
these problems, their solution is not a task that 
can be entrusted to any one group nor to any 
one science. It is a matter for physicists and 
engineers on the one hand and for psychologists 
and physiologists on the other. And some of the 
problems call for the services of still other spe¬ 
cialists, such as phoneticians and students of 
speech. 


There were employed about 40 research men 
trained primarily in acoustics, electronics, and 
related subjects and about 35 scientists whose 
specialty was psychology, physiology, or 
speech. 11 From the fall of 1940 almost until 
the end of the war the work was divided prin¬ 
cipally between two laboratories, the Electro- 
Acoustic Laboratory, under the direction of 
L. L. Beranek, and the Psycho-Acoustic Labora¬ 
tory, under the direction of S. S. Stevens. The 
philosophy of operation called for a certain 
division of labor, according to which the purely 
physical and electronic problems were pursued 
by the group at the Electro-Acoustic Labora¬ 
tory, while those problems arising from the 
fact that a human being is part of the total 
circuit were made the specialty of the Psycho- 
Acoustic Laboratory. Actually, and fortunately, 
this division was never sharply maintained, 
and before the final bomb fell some of the 
physicists were deep in psychophysics and some 
of the psychologists were engineering new 
technical devices. 

Early in 1945 a third laboratory was estab¬ 
lished to work out better methods of “passing 
the word” on shipboard, especially in the Com¬ 
bat Information Center, the nerve center of a 
ship. The Systems Research Laboratory oper¬ 
ated mainly at a special field station set up at 
Beavertail Point, Jamestown, Rhode Island, 
where radar and communication gear was as¬ 
sembled to duplicate shipboard installations. 
The research staff included about 20 research 
men, mainly psychologists and physicists, but 
also a sprinkling of specialists in the field of 
“time and motion” engineering who were 
charged with the diagnosis and remedy of 
inefficiency in the activities and movements of 
the operating personnel at battle stations. 

Partly because of the tendency of these 
projects to straddle the conventional scientific 
boundaries, and partly because they got off to 
an early start more than a year before Pearl 
Harbor, they had a varied and interesting con¬ 
tractual history with the United States Gov¬ 
ernment and its agencies. It was never entirely 
clear to which division of OSRD the Harvard 
group ought most logically to be attached. It 


a Under OSRD Contract OEMsr-658. 




THE THREE LABORATORIES 


3 


ended up with Division 17 (Physics) Sec¬ 
tion 3 (Acoustics), whose chief was Harvey 
Fletcher. On the other hand it undertook joint 
enterprises with Division 13 (Communications) 
and with the Applied Psychology Panel, and it 
might reasonably have had its primary associ¬ 
ation with either of these administrative units. 

The Harvard projects got their start when 
the Army was seeking assistance with the prob¬ 
lem of noise in airplanes. The question was not 
only how to reduce the noise in military aircraft 
but how to gauge its importance as a factor 
affecting the efficiency of pilots and aircrews. 
A Committee on Sound Control of the National 
Research Council [NRC] was organized, under 
the chairmanship of P. M. Morse, and this com¬ 
mittee supervised the early project set up at 
Harvard. Funds were supplied by a contract 6 
between the National Defense Research Com¬ 
mittee [NDRC] of the Council of National De¬ 
fense and the National Academy of Sciences on 
behalf of its agency, the National Research 
Council. A second contract 0 carried the project 
through the succeeding year and provided for 
an enlarged staff and a new program directed at 
problems of interphone communication. At the 
end of this period a new contract* 1 was written 
directly between OSRD and Harvard Univer¬ 
sity, and the NRC Committee on Sound Control 
ceased functioning. The supervision of this con¬ 
tract fell to NDRC Division C (Communica¬ 
tions), Section 9 (P. M. Morse, Chairman), 
where it remained until the final reorganization 
of NDRC in January 1942. Under this reorgani¬ 
zation the project landed in its final resting 
place, NDRC Section 17.3. 

The last-mentioned contract* 1 and its supple¬ 
ments remained in force until the end of Janu¬ 
ary 1946. It provided a continually expanding 
series of performance clauses, based on a total 
of 14 Army and Navy directives, and resulted 
in the body of research outlined in the suc¬ 
ceeding chapters. 

b Contract NDCrc-52, effective November 10, 1940, to 
June 30, 1941. 

c OEMsr-39. 

« OEMsr-658. 


The final windup caught many of the more 
basic research projects in midstream. Their 
abrupt termination would have meant the for¬ 
feit of a substantial return on a large invest¬ 
ment of public funds, for answers almost at¬ 
tained would have been more or less perma¬ 
nently lost as the research personnel scattered 
to other jobs. It was at this point that the 
newly created Office of Naval Research of 
the U. S. Navy demonstrated its wisdom 
and foresight. Organized to sponsor funda¬ 
mental research of interest to the national de¬ 
fense, this office set about to preserve those 
parts of the OSRD activities which could be 
turned toward basic long-range investigations 
of permanent significance. The entire research 
program under OEMsr-658 obviously could not 
be continued intact: it placed too large an em¬ 
phasis on immediate, practical, wartime prob¬ 
lems and it was staffed by men who had per¬ 
force to return to their peacetime occupations. 
The nucleus of basic problems stemming from 
the more practical wartime issues provided, 
however, the subject for a contract between the 
U. S. Navy and Harvard University. 6 The part 
of the program concerned with acoustics and 
electronics is continuing under Project Order I 
of this new contract, and Project Order II pro¬ 
vides for a program of psycho-acoustic studies, 
with special emphasis on basic research in hear¬ 
ing and communications. Under this contract 
the Psycho-Acoustic Laboratory is bringing to 
completion the more significant researches 
which it initiated under OSRD, and it is em¬ 
barking on fundamental inquiries not feasible 
under the stress of war. 

Under another Navy contract, with Johns 
Hopkins University, the program initiated by 
the Systems Research Laboratory is also being 
continued and extended to cover basic research 
on information systems. The Hopkins program 
is under the direction of C. T. Morgan, who 
served during the war as Technical Aide to 
NDRC Section 17.3. 


e Contract N5ori-76. 






Chapter 2 

SOUND CONTROL 


21 MEASUREMENT OF AMBIENT NOISE 

I N order to assess quantitatively the effect of 
ambient noise on military personnel and to 
devise methods of reducing acoustic stress, it is 
necessary to make physical measurements of 
the noise intensities and spectra existing in rep¬ 
resentative military environments. In the early 
phases of the research carried out at the 
Electro-Acoustic and Psycho-Acoustic Labora¬ 
tories in 1941 and 1942 it became evident that 
noise of battle has its most serious effect on 
voice communications. Combat vehicles, such as 
aircraft, ships, and tanks, are typical cases 
where the detrimental effect of ambient noise 
on voice communications is striking. The study 
of this problem formed a sizable part of the 
total effort of the two laboratories until the 
close of World War II. Several chapters are 
devoted to it. 

The effect of noise on the psychomotor effi¬ 
ciency of personnel in battle was studied at the 
Psycho-Acoustic Laboratory and forms the sub¬ 
ject matter of Section 2.5. Temporary hearing 
loss in military personnel after exposure to 
noise is quite common unless adequate precau¬ 
tions are taken (see Section 2.6). 

The problems of meaningful measurement of 
levels and spectra of ambient noise and meth¬ 
ods for their prediction and reduction have been 
studied extensively at the Electro-Acoustic 
Laboratory since 1941 and form the subject 
matter of the first four sections of this chapter. 

The noise in combat vehicles of various kinds 
shows certain typical characteristics, since the 
physical processes of its generation are of a 
similar character. It can be analyzed into the 
following general components. 

1. Noise produced by rotating or reciprocat¬ 
ing mechanisms of propulsion. The noise spec¬ 
trum consists of discrete components and their 
harmonics. The fundamental frequencies are of 
the order of 100 c. The harmonics decrease in 
magnitude with increasing order. 

2. Noise produced by impact (tank treads, 
gear transmissions) or noise produced by air 


turbulence (aircraft) and similar causes. The 
spectrum of this noise approaches the continu¬ 
ous type and is, in most cases, the chief con¬ 
tributor from medium frequencies upward. In 
general, the spectrum level- decreases with fre¬ 
quency. 

This section deals chiefly with the intensi¬ 
ties and spectra of noise in typical combat ve¬ 
hicles and the methods and apparatus used to 
measure them. Consideration of battle noise 
of other types, such as gunfire, explosions, etc., 
has been deliberately omitted. Because of the 
transient nature and violence of these phenom¬ 
ena, their measurement, their reproduction in 
the laboratory, and their quantitative evalua¬ 
tion are difficult. Available field data are scant. 


Methods and Apparatus 

The apparatus suitable for noise analyses in 
combat vehicles and elsewhere consists essen¬ 
tially of a microphone, a suitable variable band¬ 
pass filter, an amplifier, and an indicating in¬ 
strument. 

The microphone is of the wide-range dynamic 
type" which has been calibrated in terms of the 
free-field sound pressure in a random sound 
field. This is the sound pressure which would be 
measured by a “point” microphone immersed in 
a random (diffuse) sound field. This type of 
sound field corresponds fairly closely to the 
sound fields commonly encountered. 

The random incidence calibration is arrived 
at approximately by computational means, 
using free-field calibrations for progressive 
sound waves of various angles of incidence se¬ 
cured in an anechoic chamber of sufficient qual¬ 
ity. All measured sound-pressure levels are 
expressed in decibels referred to the ASA refer¬ 
ence level 10 of 0.0002 dyne/cm 2 . 

The variable band-pass filter covers the audio¬ 
frequency range from about 30 to 10,000 c. To 
analyze the various discrete low-frequency con- 

a Western Electric instruments Types 630-A and 
633-A have been used successfully. 


4 



MEASUREMENT OF AMBIENT NOISE 


5 


tributions to the noise spectrum, a continuously 
variable filter arrangement of the wave-analyzer 
type can be used. Such an analysis, while time- 
consuming, permits interpretation of the vari¬ 
ous prominent spectral components in terms of 
their respective sources. A graphic analysis of 
the spectrum of a bomber treated with sound¬ 
absorbing material is shown in the top half of 
Figure 1. Since a filter of a bandwidth of 5 c 


effect of low-frequency noise components (be¬ 
low about 200 c) in voice communications in a 
wide-band noise is, in many cases, compara¬ 
tively small. It has been found preferable in 
most cases to use a filter set variable in con¬ 
venient discrete steps. A filter set covering the 
range from about 40 to 9,000 c in eight octaves 
has been found adequate and useful. The analy¬ 
sis of the noise spectrum mentioned above with 


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Figure 1. Noise in multi-engine bomber, treated with absorbing material. Top—analysis with narrow 
band-pass filter. Bottom—analysis in octave bands. 


has been used, enough resolution was obtained 
to distinguish the noise components produced 
by the engine, propeller, and exhaust. 

In many instances the type of analysis men¬ 
tioned above is too time-consuming. Further¬ 
more, the detailed information afforded in the 
low frequencies is often not necessary, since the 


the octave filter set is shown in the lower part 
of Figure 1. An “overall” position, with no 
filters in the circuit, is added. 

The indicating instrument is of the square- 
law type and is calibrated in decibels. A meter 
with pointer indication and a graphic level re¬ 
corder are provided. The recorder is used more 



































































6 


SOUND CONTROL 


frequently in field work where automatic opera¬ 
tion is desirable. 

An automatic octave sound analyzer and re¬ 
corder designed by the Electro-Acoustic Labora¬ 
tory is shown in Figure 2. This system is com¬ 
posed of the following parts: 

1. The microphone. 

2. The octave band-pass filter set. 

3. The sound-level meter (containing the am¬ 
plifiers and the indicating meter). 


up the required flight conditions. Pressure on 
the pushbutton of the control unit sets the 
device in operation. For each of the nine filter 
positions in succession, the sound level is re¬ 
corded automatically by the level recorder on 
waxed paper tape. The filter bands are changed 
automatically by a relay. After all nine bands 
have been used, the device is ready for another 
analysis which is indicated to the pilot by a 
light signal. 



Figure 2. Automatic octave sound analyzer with recorder. 


4. The graphic level recorder. 

5. The hand-held remote control unit. 

6. The battery power supply. 

The equipment was specially designed for 

automatic operation, portability, and compact¬ 

ness. It is a matter of relative ease to install it 

in the rear compartment of a single-seater air¬ 

plane and place the control unit in the hands of 

the pilot. After the microphone has been placed 
at the desired position and after adjustment and 
calibration of the apparatus, the pilot can set 


The case shown at the left in Figure 2 con¬ 
tains the filter set, sound-level meter and bat¬ 
teries, and weighs about 125 lb. Its dimensions 
are 19 x 18 x 11 in. The second case contains the 
recorder and the rest of the batteries. It weighs 
about 105 lb and has the dimensions 19 x 18 x 12 
in. In case space is very limited, the units can be 
broken down into their component parts, as 
shown in Figure 3. This equipment is described 
in detail in a report 4 5 6 * * * * * 12 issued by the Electro- 
Acoustic Laboratory. That laboratory has also 



MEASUREMENT OF AMBIENT NOISE 


7 


assisted the Services in writing the Army-Navy 
Specifications AN-N-7 for noise-measuring 
equipment, in which further details may be 
found. 

The data presented in the following sections 
were taken using equipment of the type de¬ 
scribed. 


Noise in Airplanes 

The most prominent low-frequency compo¬ 
nents in the noise spectrum of airplanes are 


2. Propeller-tip speed. 

3. Minimum separation of propeller tips from 
fuselage. 

4. Exhaust type. 

5. Airspeed. 

6. Type of airplane and location of cabin. 

7. Transmissive and absorptive properties of 
the cabin walls. 

8. The quality of seals around windows, 
hatches, and the type of cabin ventilation used. 

Noise analyses have been carried out by the 
Electro-Acoustic Laboratory in more than 
thirty-five different types of military aircraft 



Figure 3. Sound analyzer—component parts. 


directly related to the propeller-tip passage fre¬ 
quency and its harmonics. The noise of the con¬ 
tinuous type at medium and high frequencies is 
due to the aerodynamic noise of the air rushing 
past the fuselage and to turbulence noise pro¬ 
duced largely at the propeller tips. The noise 
spectrum and levels inside an aircraft cabin 
depend 'primarily upon the following factors: 

1. Engine power. 


under a variety of flight conditions. A number 
of reports 180 - 21 • 25 - 31 have been issued on the 
subject. 

Sample spectra for a number of aircraft of 
different types are shown in Figures 4 and 5. 
It is evident that, in modern, high-speed, high- 
powered, single-engine aircraft, noise conditions 
are more severe than in large multi-engine 
bombing craft. As a rule, only multi-engine 







8 


SOUND CONTROL 


airplanes afford space and opportunity for in¬ 
stallation of sound-absorbing treatment. This 
can materially reduce the sound levels at me¬ 
dium and high frequencies and results in im¬ 
provement of voice communications. 

Note that in the jet-propelled airplane (see 
Figure 5) the absence of a propeller and con- 


spectrum changes with varying engine power 
conditions. As the propeller-tip velocity reaches 
high values, the high-frequency components of 
the spectrum increase more and more relative 
to the low-frequency components. An analysis 
with a narrow-band analyzer shows that the 
harmonics of the propeller-tip passage fre- 




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Figure 4. Noise levels and spectra in aircraft. 


ventional engine results in low sound levels in 
the low-frequency bands. All spectra shown are 
for airplanes which are not treated by installa¬ 
tion of sound-absorbing material. 

Figure 6 shows how the noise level and 


quency increase very rapidly under those con¬ 
ditions. 

As a further illustration of the many vari¬ 
ables entering into the picture, consider the 
influence of altitude on the noise levels and 







































































































































































MEASUREMENT OF AMBIENT NOISE 


9 


spectra. The differences in the sound levels 
measured due to a change in altitude in octave 
bands are plotted for a P-47D and a B-17F air¬ 
plane for constant engine power in Figures 7 
and 8. The changes in spectrum and level are 
slight even for altitudes near the ceilings of 
the aircraft involved. 


« 

is shown by the contours for a PBM-3 airplane 
in Figure 10. 

To obtain an estimate of the sound levels gen¬ 
erated by the firing of machine guns in a typi¬ 
cal bomber, such as the B-17F, the microphone 
was placed at the top turret gunner’s knees. A 
reproduction of the record obtained during gun- 


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Figure 5. Noise levels and spectra in aircraft. 


The variations of the levels inside a cabin are 
illustrated by the contours of equal sound levels 
for a DC-3 transport airplane, given in Figure 9. 

How an imperfectly sealed window can in¬ 
crease the levels in certain bands in its vicinity 


nery practice is shown in Figure 11. Instan¬ 
taneous increases in the overall sound level of 
10 db or more were noticed. 

The noise levels at high frequencies can be 
materially reduced by proper installation of 






























































































































































10 


SOUND CONTROL 


sound-absorbing material. The added weight 
can be kept within comparatively small limits. 
Figure 12 gives an estimate of the reduction of 
noise levels which was realized by the experi- 



OVER- <75 75 150 300 600 1200 2400 >4800 
150 300 600 1200 2400 4800 


FREQUENCY OCTAVES 
(Cycles per second) 


Figure 6. Noise levels in a B-24D bomber as a 
function of engine power. 


mental installation of a small amount (about 
40 lb) of additional absorbing material in a 
Mitchell type B-25C bomber. Although the 
lower octave levels and the overall levels are, in 


proper design and installation of acoustical 
treatment is discussed in Section 2.3. 


2,1-3 Noise in Tanks and Landing Vehicles 

In Figure 13 noise spectra and levels meas¬ 
ured inside the turret of a medium and light 
tank are given. Among the chief sources of 
noise are the clatter of sprockets and treads and 
the engine. Although the noise measurements 
discussed here were made early in World War II, 
the spectra found in tanks of later design are 
not likely to be very different in character. 
There is every reason to believe, however, that 
the levels will be at least as high as shown in 
Figure 13, if not higher. Increased engine 
power, speed, and fire power all make for in¬ 
creased noise. Judicious application of sound¬ 
absorbing materials, vibration insulation of 
critical parts, and replacement of steel treads 
with rubber treads can markedly reduce the 
noise levels in the important bands. As an ex¬ 
ample, the noise in a Navy Amphibious Landing 
Vehicle (LVT-4)- 4 is analyzed and measures for 
its reduction are described below. It should be 
emphasized that proper preproduction acoustic 
engineering and design is preferable to any 
postproduction modifications. 


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Figure 7. Effect of altitude on the sound levels 
in a P-47D airplane for constant engine power. 


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Figure 8. Effect of altitude on the sound levels 
in a B-17F airplane for constant engine power. 


general, not markedly affected by the treat¬ 
ment, the reduction of the levels in the upper 
octaves may yield marked improvement in the 
performance of the interphone aboard. The 


Figure 14 shows the noise levels and spectra 
in the (untreated) cab of an LVT-4. The levels 
are among the highest found in any combat 
vehicle. Voice communication under these con- 


























































































MEASUREMENT OF AMBIENT NOISE 


11 


ditions is very difficult and temporary deafness sources indicated in detail. In passing, it may be 
of combat personnel is common. noted that in the present case the exhaust con- 

The sources of noise contributing to the levels stitutes the chief contributor of low-frequency 


DC-3 



75-150 C 
OCTAVE BAND 



1200 - 2400 C 
OCTAVE BAND 

Figure 9. Contours of equal sound levels in a 
DC-3 airplane. 

in the important bands are, in order of decreas¬ 
ing importance, 

1. Forward sprocket drive. 

2. Engine. 


MARINER PBM-3 



OCTAVE BAND 

Figure 10. Contours of equal sound levels in a 
PBM-3 airplane. 

3. Transmission. 

4. Chain, rear sprocket, exhaust, etc. 

A cross section through the vehicle is shown 
in Figure 15, with the location of the noise 


CD 125 




Figure 11. Transient noise levels due to firing 
of two machine guns in a B-17F airplane. 


noise. Introduction of a suitable muffler results 
in considerable improvement. 

By application of sound-absorbing material 



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Figure 12. Effect of experimental installation 
of sound-absorbing material in a B-25C airplane. 

to the interior of the cab and by replacing the 
forward-drive sprockets with sprockets having 







































































































































12 


SOUND CONTROL 


rubber vibration insulators, the noise can be 
reduced. Figure 14 shows that reductions in 
noise level of the order of 10 db are realized. 


Noise on Ships and Boats 

The sources of noise on warships are of a 
varied nature, especially at exposed stations. 


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Figure 13. 


75 150 300 600 1200 2400 >4800 
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FREQUENCY OCTAVES 
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Noise levels and spectra in tanks. 


Gunfire, airplane noise on aircraft carriers, and 
wind are among the most important ones. There 
are, however, also important stations below 


deck where the ambient noise is considerable. 
As an example, the noise spectrum in the en¬ 
gine room of a submarine is shown in Figure 
16. The levels are considerable and communica¬ 
tion is seriously interfered with. 



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Figure 14. Noise levels and spectra in an 
LVT-4 landing vehicle. 

In certain situations in World War II it was 
necessary to land military personnel in small 
boats equipped with outboard motors on enemy- 
held shores. It was of vital importance to escape 
detection. In order to evaluate quantitatively 



Figure 15. Main sources of noise in an LVT-4 
landing vehicle. 

the sound levels produced by a typical outboard 
motor (10 to 20 hp) measurements were made 30 
by the Electro-Acoustic Laboratory. A highly 
directional tubular microphone was used which 
was mounted on shore. The boat was held sta¬ 
tionary at a distance of about 45 ft. (See Figure 
17.) By rotating the boat, a directional diagram 





































































































































MEASUREMENT OF AMBIENT NOISE 


13 


of the sound source could be obtained. A dummy 
propeller was used and the motor adjusted to 
rated power conditions. Figure 18 shows the 
directional pattern of the motor noise. It ap¬ 
pears from the diagram that the radiation in 



OVER- <75 75 150 300 600 1200 24004800 

ALL 150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 


Figure 16. Typical noise levels and spectra in 
the engine room of a submarine. 

the direction of the bow is comparatively weak. 
Radiation is strongest in a direction perpendicu¬ 
lar to the boat’s course. The solid line in Figure 
19 shows the noise spectrum of a typical out¬ 
board motor. The dashed line was obtained after 



Figure 17. Experimental determination of di¬ 
rectional characteristics of outboard motor noise. 


ness of treatment. This distance is about 300 to 
500 yd at full speed but depends strongly on the 
prevailing wind, among other things. 




150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 


Figure 19. Noise levels and spectra of a typical 
small outboard motor. 


installation of a jacket of sound-absorbing ma¬ 
terial and suitable intake and exhaust silencers. 
Jury tests of the maximum detectable distance 
of the approaching boat confirm the effective- 


2,1,5 Miscellaneous 

In this section miscellaneous sound-level 
measurements are discussed which were carried 



























































































14 


SOUND CONTROL 


, 1 1 1 ... 

| KAYT JOBSSTP K-2k } 



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FREQUENCY OCTAVES 
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Figure 20. Comparison of noise spectra in a 
type K airship and a DC-3B airplane. 


out by the Electro-Acoustic Laboratory and 
which are of more than passing interest. 

Sound levels and spectra in Navy Type K 
airships have been investigated extensively and 
the findings were published in a report. 29 The 
sound levels in the cabin are comparatively low, 
even without sound-absorbing material. Figure 
20 shows the envelope for all crew positions for 
a variety of flight conditions. For comparison, 
the spectrum for a DC-3B transport plane has 
been added in the figure. The noise conditions in 
the two ships are, on the average, very nearly 
alike. 

Figure 21 shows a section through the air¬ 
ship car with some of the crew positions indi¬ 
cated. It is of interest to study the change in 
sound levels as the test microphone is carried 
along the cab as a function of the distance from 
the propeller plane. The results of such meas¬ 
urements for two constant power conditions are 



GUNNE 


Figure 21. Crew positions in cab 


FRAME #1 


of Type K airship. 


NAVIGATOR 
#6 

RADIO 
#5 


L 

L -PROPELLER PLANE 


PILOT 

#7 

















































MEASUREMENT OF AMBIENT NOISE 


15 


plotted in Figure 22. The ordinate is the differ¬ 
ence, in decibels, of the average levels in all 
bands for a given distance from the propeller 
plane and the average levels measured in the 
propeller plane. 

The sound levels due to an airplane passing 
overhead are of interest in certain questions of 



Figure 22. Variation of noise levels with dis¬ 
tance from propeller plane in cab of Type K air¬ 
ship. 


countermeasures against low-level attack by 
aircraft. The Electro-Acoustic Laboratory car¬ 
ried out a series of measurements of the sound 
levels at ground level due to a medium bomber 
flying overhead in level flight at a specified alti¬ 
tude. A B-18 medium bomber was made avail¬ 
able for this purpose. 

A dynamic microphone equipped with a wind¬ 
screen was placed on the ground at the airport 
where the tests were performed. 15 It was con¬ 
nected through suitable amplifiers to a Miller 
film recorder. The pilot was instructed to fly 
the airplane in a straight course over the micro¬ 
phone location, maintaining a constant altitude 
and specified flight conditions. 

The sound picked up by the microphone was 
recorded as a function of time for various alti¬ 
tudes and flight conditions. A reference time 
mark was introduced by shorting out the signal 
when the craft was directly overhead. 

The Miller film recording method has been 
described in the literature. 1 The recording me¬ 
dium is a narrow film tape, consisting of a 
celluloid base and a layer of transparent soft 
gelatin covered with a thin, uniform layer of a 
chemical opaque to light. The recording head 


consists of a wedge-shaped stylus driven by an 
electromagnetic drive. The stylus removes the 
opaque layer in varying widths, corresponding 
to the wave form of the signal. Reproduction is 
effected by conventional optical means. Figure 
23 shows a sound track for airplane noise. A 
60-c wave has been recorded for comparison. 

The records obtained in the field were played 
back and analyzed in the laboratory. Two meth¬ 
ods were used. 

1. The playback signal was analyzed by a 
variable band-pass filter whose output signal 
was recorded by means of a high-speed-level 
recorder. By successive playbacks, the levels in 
each octave band were recorded as a function 



Figure 23. Recording of airplane noise on 

Miller film. 

of time. Figure 24 shows a set of such records 
for maximum power conditions and an altitude 
A of 100 ft. The dip in each record corresponds 
to the time mark indicating the airplane to be 
overhead. H is the horizontal distance of the 
airplane from the vertical through the micro¬ 
phone location. For convenience, the maximum 
levels in each band have been plotted in Figure 
25, with altitude as parameter. It is evident 
from Figure 24 that in the first octave the 
maximum sound level is attained after the air¬ 
plane has passed the microphone location by 
several hundred feet. This points to the well- 
known existence of maxima in the radiation pat¬ 
tern near the propeller plane. 

2. A more detailed frequency analysis was 
afforded by the use of a narrow-band (20 c 
wide) wave analyzer. The film containing the 
record of one flight was pasted together to form 
a loop which made repeated playback con- 















16 


SOUND CONTROL 


TIME 


i: Vm • « • • . «'• . 

Overall ^ - - 


-Z 




2= 




3k 




70 _ [-i | i n 

t 

;— 

■o 

“I -1 


+ H 

3000 2000 1JOC 


2 

4 

1000 

2000 

3000 Feet 


TJ — 

C 100- 

"I 90- 


. ; ^j“i^vv o T , 7TT rr» j • t * • 


50 - f - 85 c 








3 70 
o 
SO 


zT"' I 1 -l 1 t 
1000 <? 


3000 2000 


* 1 I 1 'I 1 I +H 

2 1000 2000 3000 Feet 





170 -f- 340 c 


E 


- _Aa^M^^—C 




3 70 
o 
<0 


z'7~' I 1 I 1 J 

3000 2000 1000 J 


• i + H 

8 1000 2000 3000 Feet 



B-18 MAXIMUM POWER I80mph IAS 

ALTITUDE 100 FT 1000 hp 2100 rpm 

Figure 24. Sound levels due to an airplane 
passing overhead as a function of horizontal dis¬ 
tance H. 


venient. For each center frequency of the filter 
pass band the maximum sound level was read 
on the analyzer and plotted as a function of 
frequency in Figure 26. Good correlation of the 



85 170 340 680 1360 

FREQUENCY OCTAVES 
(Cyclfls per second) 


Figure 25. Maximum sound levels due to a B-18 
bomber passing overhead. 



Figure 26. Maximum sound levels in 20 c bands 
due to a B-18 bomber passing overhead (altitude 
100 ft). 


peaks in the spectrum with the propeller-tip 
passage frequency and its harmonics was ob¬ 
tained. 






































































































































































































REDUCTION OF AMBIENT NOISE AND VIBRATION 


17 


For more accurate work doppler corrections 
and corrections due to the finite sonic velocity 
would have to be applied. 1 

In aircraft factories and similar establish¬ 
ments noise levels may be so severe as to lead 
to temporary and, after long exposures, per- 


are an attempt to indicate the range of the 
fluctuations. The noise levels are seen to be 
particularly severe at high frequencies, where 
hearing loss is most easily incurred. Effective 
aural protection for the operating personnel is a 
necessity under such conditions. 



150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 



150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 


130 


120 
UJ 

> — 

<t CM 

£ E no 

o u 
o ^ 

V 

a: c ioo 

uj >. 

a. -o 

gS 90 
o 

z o 

_J °* 80 

UJ 11 

UJ > 

_l ® 


70 


O H- 

z « 
o a: 
o — 
co 


60 

50 


40 



OVER- <75 75 150 300 600 1200 2400 >4800 
ALL 150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 


(Cycles per second) 



150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 


Figure 27. Noise levels and spectra at various stations in an aircraft factory. 


manent hearing losses of the personnel involved. 
Figure 27 gives representative noise spectra and 
levels near various noise sources in an aircraft 
factory. 14 Many noises encountered there are of 
a transient, impulsive nature and the levels vary 
considerably as a function of time. In such in¬ 
stances the cross-hatched areas in Figure 27 


2 2 REDUCTION OF AMBIENT NOISE 
AND VIBRATION 

2-2,1 General Considerations 

A typical section of the fuselage in an air¬ 
plane which is treated with absorbent material 




































































































































18 


SOUND CONTROL 


consists of the following two parts: the outer 
skin, made of Dural, and the structure contain¬ 
ing the acoustic material, separated from the 
skin in most cases by an air space an inch or 
two in depth. Sound originating outside the air¬ 
plane and striking the Dural skin w r ill be par¬ 
tially transmitted into the interior of the 
cabin. A sound field will be set up in the in¬ 
terior of the cabin which will, in part, be 
determined by the fact that some of the sound 
inside the cabin will be absorbed by the acoustic 
material. Although this description is some¬ 
what crude and the physical processes of trans¬ 
mission and absorption are not easily separable, 
it has been found helpful, nevertheless, to sep¬ 
arate them in practice and to define a transmis¬ 
sion coefficient x and an absorption coefficient a. 
The transmission coefficient x is defined as the 
ratio of the sound energy transmitted through 
the acoustic structure to that incident upon it. 
It is often more convenient to express x in terms 
of the attenuation N in decibels, according to 
the following relation 

—.V/ 10 

T = 10 . (1) 

The absorption coefficient a is defined as the 
ratio of the sound energy absorbed by the 
acoustic structure in question to that incident 
upon it from the cabin inside. Although these 
definitions are useful for the purpose of dis¬ 
cussion, a meaningful experimental evaluation 
of x and a will require restatement and modifi¬ 
cations. This is discussed in Section 2.2.3. 

To elaborate somewhat on the general pic¬ 
ture, consider an enclosure, say an airplane 
cabin, immersed in a sound field of uniform 
sound energy density. Assume further that the 
walls of the cabin are uniform and that the 
structure has a transmission coefficient x and an 
absorption coefficient a. If it is assumed that 
the resultant sound field inside the cabin is also 
of uniform sound energy density, it can be 
shown that the reduction in sound level due to 
the walls equals 10 log (x a) /x, in decibels. 
A plot of this function is shown in Figure 28. 
Contours of constant reduction in sound level 
are plotted as a function of a and x. The chart is 
not applicable to practical cases except in broad 
terms, due to oversimplification of the underly¬ 


ing assumptions. It does, however, reveal the 
important fact that appreciable values of sound- 
level reduction are possible only if a is appreci¬ 
able and x is small. 

Clearly, it is impractical to determine the 
sound-level reduction by direct experimental 
means in an aircraft. Considerable research has 



Figure 28. Theoretical reduction of sound levels 
in a cabin whose walls have a transmission 
coefficient x and an absorption coefficient «. 


shown that laboratory measurements under cer¬ 
tain specified conditions yield values related to 
the absorption coefficient a and transmission 
coefficient x, as defined above, which have been 
successfully used in predicting the actual per¬ 
formance of a structure. 

In the following, a and N (and x) will be used 
only to denote quantities obtained by a specified 
apparatus in a specified manner in the labora- 








































REDUCTION OF AMBIENT NOISE AND VIBRATION 


19 


tory. b For the details of procedure and appara¬ 
tus, Section 2.2.3 should be consulted. 

The process of sound absorption in porous 
materials can be described roughly as follows: 
Sound waves impinging on the material force 
air in and out of the channels between the 
fibers of the material, where energy dissipation 
due to friction takes place. In materials not con¬ 
taining an impervious septum, air can be forced 
through at a steady flow. It can be shown that 
the resistance to steady flow, the so-called flow 
resistance R, of a material can be correlated 
with its absorptive properties. 

Weight, thickness, and the space configura¬ 
tion of the components of a structure are other 
important parameters in determining the effec¬ 
tiveness of a structure as an acoustical barrier 
to transmitted sound and an absorber of sound 
inside the enclosure whose walls are formed by 
that structure. There are, for a given struc¬ 
ture, optimum combinations of these param¬ 
eters. 

The emphasis placed on aircraft in this and 
the following sections is, for the most part, due 
to the relative importance of aviation in World 
War II. It is not intended to imply (1) that 
noise conditions are unimportant in other com¬ 
bat vehicles (see Section 2.1.3) and (2) that the 
results of the research and methods for noise 
prediction and reduction are not applicable to 
cases other than aircraft. 

Anticipating the results of later sections, it is 
the medium- and high-frequency noise com¬ 
ponents which seriously interfere with com¬ 
munications. In view of this fact, it is in general 
not worthwhile to try to reduce the low- 
frequency vibrations of windows, panels, etc., 
in aircraft. Their action as secondary sound 
sources at low frequencies may result in some 
increase of the sound levels at certain low fre¬ 
quencies. It is, in general, more efficient to 
forego special vibration treatment of such 
panels and windows in favor of design for 
maximum noise reduction for medium and high 
frequencies. 

In conclusion, a word should be said with 
reference to the problem of vibration to which 

b This is not strictly ti'ue with regard to a, since an 
estimate of the values obtained by two different experi¬ 
mental techniques was found most useful. 


military personnel is exposed in aircraft and 
other combat vehicles. Reduction of excessive 
vibrations sometimes encountered at certain 
crew positions can be effected by more or less 
conventional techniques. An example of vibra¬ 
tion insulation of a pilot’s seat is given in a 
report 8 compiled by the Electro-Acoustic 
Laboratory. 

Vibrations affecting structural design are be¬ 
yond the scope of this discussion. 


2 ' 2 ' 2 Properties of Acoustic Materials 
and Structures 

The general requirements which an acoustic 
material for sound control in combat vehicles 
must meet are as follows. 

1. It must attenuate sound effectively in the 
important frequency range. 

2. It must absorb sound effectively in the im¬ 
portant frequency range. 

3. It must be light in weight. 

4. It must have low thermal conductivity. 

5. It must have high flame resistance. 

6. It must have high corrosion resistance. 

7. It must have adequate mechanical 
strength. 

8. It must be vibration-proof. 

9. It must be moisture resistant and quick¬ 
drying. 

10. It must resist rot and fungi. 

11. It must be vermin-proof. 

Sound Attenuation 

The simplest structure consists of a single 
impervious septum, say, the outer Dural skin 
of an airplane. At low frequencies it is possible 
to calculate the attenuation N of such a struc¬ 
ture from the physical properties of the septum 
and the mounting conditions. In the vicinity of 
the normal modes of vibration N is found to be 
very small. Between resonances, the attenuation 
is little greater than the values resulting from 
the assumption that the panel can be replaced 
by a pure mass with zero stiffness. At frequen¬ 
cies above several hundred cycles per second the 
average attenuation can be predicted from the 
surface density a of the panel alone. 18 If uniform 
sound-pressure distribution over one side of the 





20 


SOUND CONTROL 


panel is assumed and if it is further assumed 
that the panel is terminated on the other side 
by a perfectly absorbing termination, 

N « 20 log ^ (2) 

no 

if N is identified with the ratio, in decibels, of 
the sound pressures in front and in back of the 
panel. 

<7 = Surface density of panel in grams per 
square centimeter, 
co = 2irf (/, frequency in cycles). 

R o = 42 grams per square centimeter per 
second. 

The expression R„ is the specific acoustic im¬ 
pedance of the perfectly absorbing termination, 
equivalent to free air. Equation (2) in this con¬ 
nection is often referred to as the weight law. 



Figure 29. Theoretical values of attenuation N 
vs frequency for weight law. 

Figure 29 shows computed values of the at¬ 
tenuation N, in decibels, for various values of 
surface density a. 

Figure 30 shows an experimental curve of N 
vs frequency for a Plexiglas window obtained 
on the attenuation measuring apparatus to be 
described in Section 2.2.3. The appropriate 
weight law curve is drawn in, for comparison, 
from Figure 29. Note the prominent individual 
resonances, especially at low frequencies. The 
fluctuations and irregularities are considerable 
everywhere. But it is clear from inspection that 
the weight law curve seems to approximate the 
average value of N at medium and high fre¬ 


quencies reasonably well. The weight law will 
be used as a practical yardstick of comparison 
of the attenuating properties of structures. 

As an example of such a structure, consider 



Figure 30. Attenuation of a single Plexiglas 
window. 


Figure 31, on which A 7 ’ for a double impervious 
septum (double Plexiglas window) is plotted vs 
frequency. This structure is considerably 
“better” than the weight law. 



30 IOO 1000 10.000 


FREQUENCY IN CYCLES PER SECOND 

Figure 31. Attenuation of a double Plexiglas 
window. 

From investigations of various structures 
consisting only of impervious septa the follow¬ 
ing conclusions can be drawn with respect to 
their attenuation properties (sound absorption 
by these structures is very small) : 

















































































































































































REDUCTION OF AMBIENT NOISE AND VIBRATION 


21 


1. Impervious septa are, at low frequencies, 
almost transparent to sound. The low-frequency 
resonances can be damped by cementing or 
spraying suitable damping materials on the 
septum. The practical value of such treatment is 
controversial, since the transmission of sound at 
medium and high frequencies is not appreciably 
affected. 

2. The average attenuation at medium and 
high frequencies of single septa (of sufficient 
area) can be predicted from the weight law. The 
controlling factor is the surface density. Mount¬ 
ing conditions, size, shape, etc., have negligible 
influence. 

3. Double septa attenuate sound better than 
the weight law would predict at medium and 
high frequencies. At low frequencies they afford 
no appreciable advantage. In practice, some ad¬ 
vantage may be lost owing to the necessity of 
using heavier frames for the double septum. 
Comparisons should be made on the basis of 
equal total weight. 

4. There is no advantage in supporting win¬ 
dows in elastic mountings. 

Absorptive structures consist of at least one 
layer of sound-absorbing material and one or 
more impervious septa. A large number of struc¬ 
tures are discussed in detail in Report OSRD 
1543. 18 Equivalent electric circuits can be drawn 
which describe the general behavior of struc¬ 
tures at low frequencies. Numerous examples 
are given in the report referenced above. 18 The 
attenuation vs frequency characteristics of 
single and multiple absorptive structures are 
similar in general character to the characteris¬ 
tics shown in Figure 31. The average attenua¬ 
tion values at medium and high frequencies are 
always greater than the values predicted by the 
weight law. Tests have shown that among the 
many possible structures the three configura¬ 
tions shown in Figure 32 are among the most 
effective. Structure V is the configuration rec¬ 
ommended in the Army-Navy Aeronautical 
Specifications for Sound-proofing Materials 
AN-S-32 a and AN-S-33 a . 

It is desirable to have a measure of the ef¬ 
fectiveness of structures with respect to their 
ability to attenuate sound. It has been suggested 
to take the amount by which the average atten¬ 
uation of the structure exceeds the attenuation 


predicted by the weight law at a frequency of 
5,000 c. To take into account the weight of dif¬ 
ferent materials, the difference in attenuation, 



Figure 32. Effective single absorptive struc¬ 
tures. 


n 5 , is divided by o s , the surface density of the 
total structure exclusive of the Dural skin. A 
merit factor is therefore defined as follows: 

Merit Factor [MF] = - . (3) 

0's 

The usefulness of the MF as a basis of com¬ 
parison between different structures lies in the 
fact that it affords a rank-ordering of the effec¬ 
tiveness of the structures to attenuate sound 
per unit weight. A quantitative comparison 
other than rank-ordering is in general not pos¬ 
sible. It should be remembered, however, that 
in addition the weight of the structure has 
always to be considered. 

Acoustic materials per se may be rank- 
ordered by use of the MF when placed in a 
given structure. Tests have shown that Struc¬ 
ture III (see Figure 32) has a reasonably con¬ 
stant MF for a given acoustic material inde¬ 
pendent of its thickness T. For this structure, 
the MF ranges between about 15 and 135 for 
various acoustic materials. 

A detailed discussion relating merit factor 
and the physical characteristics of the material 
and the attenuating properties of structures 
containing this material is given in Report 
OSRD 1543. 18 

It is shown there that an efficiency index E 
can be chosen which affords the possibility of 
rank-ordering different acoustic materials with 
respect to their ability to attenuate sound. The 



















22 


SOUND CONTROL 


efficiency index E is directly related to the MF 
and is given by the following equation. 



where R = flow resistance, 

T = thickness of material, 
x • • • ranges from 0.2 to 1.1 and is a 
function of the (lightweight) ma¬ 
terial, 

o m = surface density of material alone. 

To a good approximation, E is a characteristic 
constant for a given material for values of a m 
below about 0.15 lb per square feet. 

It can be shown that, for sound treatment in 
aircraft, Structure V is usually somewhat supe¬ 
rior to Structure III (see Figure 32). In Report 
OSRD 1543 18 information is supplied which aids 
in the selection of the most efficient structure of 
least surface density to achieve a given value of 
attenuation. After the structure has been 
chosen, procedures are outlined to find the mini¬ 
mum surface density o s which w T ill yield the de¬ 
sired attenuation. These problems come up in 



Figure 33. Measured flow resistance for various 
materials. 


connection with the design of sound treatment 
for an airplane which is required to reduce the 
levels in certain frequency bands below the 
maximum levels specified in the Army-Navy 


specifications. The matter of predicting the 
sound levels in the airplane before and after 
treatment and choice of structure is discussed 
in Section 2.2.3. 

Sound Absorption 

If a porous absorbent material is placed in a 
sound field, the sound pressure will tend to 
move the air particles contained in the pores 
of the material. Because of the frictional re¬ 
sistance between the air particles and the par¬ 
ticles of the material some of the sound energy 
will be dissipated. It is evident that if this 
process is to take place efficiently, the material 
itself must be heavy enough to remain station¬ 
ary, as a whole, or it must be mechanically 
constrained. The flow resistance of the material 
must be neither too small, which is an indication 



CT m IN LBS/FT 2 


Figure 34. Measured flow resistance vs surface 
density for constant thickness. 

of insufficient frictional resistance, nor too 
large, in which case the particle motion would 
be too small. It can be shown that for a given 
surface density a m of the material there is an 
optimal value of flow resistance R for maximum 
sound absorption. 

Experiments have shown that the following 
conclusions can be drawn. 

1. For efficient sound absorption at low fre¬ 
quencies, the spacing between the absorbed 
material and the Dural skin should be large. 





































































































SECTION B-B 


DETAIL A 



Figure 35. Apparatus for measuring acoustic attenuation 










































































































































































































































































































































































































REDUCTION OF AMBIENT NOISE AND VIBRATION 


23 


2. The flow resistance should be less than 
500 cgs units (g per sq cm per sec) for 1 in. of 
thickness. 

3. The trim cloth covering the cabin side of 
the structure should have a flow resistance less 
than about 40 cgs units. 

Flow Resistance 

The flow resistance R of a porous material is 
defined as follows: 


in grams per square centimeter per second, 
where <p = steady pressure drop across the 
sample in dynes per sq cm, 

V = volume (cu cm) of air passing the 
sample in t sec, 

v = velocity of flow through sample, 
A = area of sample in sq cm. 

Since the flow resistance is a measure of the 
friction occurring between the fibers or par¬ 
ticles of the material and the air passing 
through the channels between them, R depends 
upon the size, shape, and orientation of the 
channels and on the total surface area of the 
fibers in the material. These quantities depend, 
in turn, upon the diameter and cross-sectional 
shape of the fibers, the total number of fibers in 
the sample, and the manner in which the fibers 
are distributed and oriented in the material. 
One or more of these variables can usually be 
controlled in the process of manufacture of an 
acoustic material, and thereby R may be set at 
the required optimum value. 

The general relation governing the steady 
flow of a compressible gas through a porous 
medium has been derived. 4 From this general 
theory one may derive a number of simple rela¬ 
tions between the flow resistance R of a mate¬ 
rial and its physical characteristics, such as 
thickness, volume density, surface density, 
fiber diameter, and manner of construction 
(“felting”). 

The results of a large number of measure¬ 
ments of flow resistance carried out on a large 
number of lightweight acoustic materials show, 
however, that there are, in many cases, impor¬ 
tant deviations from the relation predicted by 
theory. 

Figure 33 shows the empirically obtained re¬ 


lation between R and T for several materials. 
These relations can be approximated by the ex¬ 
pression 

RT V = constant. (6) 

The exponent x takes on a characteristic 
value for each material and surface density 
<r m . The expression RT e is, by equation (4), 
directly related to the efficiency index E for 
sound attenuation, discussed previously. 

Figure 34 shows the empirical relation be¬ 
tween flow resistance R and surface density 
a,a, for a constant thickness T equal to 1 in. 
The curves were obtained by varying a m for 
a given material by packing more or less of 
it into samples of constant area. The variation 
of R under these conditions can be expressed 
as follows: 

R<j, n ~ a+X) = constant, (7) 

where x is the same as in equation (6). 

For small values of a m (below about 0.15 lb 
per square foot) equation (7) can be approxi¬ 
mated by 

— ~ constant. (8) 

a m 

Equations (6) and (8) form the experimental 
basis for the validity of E in equation (4) as a 
fixed number for a given (lightweight) ma¬ 
terial. 


Test Methods and Apparatus 

Measurement of Sound Attenuation 

In a report 32 recently published, a detailed 
description is given of the experimental evalua¬ 
tion of the sound-attenuating properties of 
structures designed for quieting aircraft. The 
method and apparatus described therein have 
proved useful and are, for the present purpose, 
preferable to the older methods described in the 
literature. In Section 2.2.2 the weight law 
[equation (2)] is arrived at by identifying the 
attenuation N in decibels with the ratio of the 
sound pressures in front and in back of the 
panel in decibels. Uniform pressure distribu¬ 
tion on the “primary” side and a perfectly 
absorbing termination on the “secondary” side 






24 


SOUND CONTROL 


were assumed. By extension, N is defined in 
the same way for more general structures 
where the weight law does not necessarily hold. 
The test equipment discussed in reference 32 is 
designed to measure the pressure ratio at the 
two sides of a structure under conditions 
approximating as closely as possible the 
assumptions made above. Residual discrepan¬ 
cies are removed by “calibrating” the equip¬ 
ment with a structure consisting of a single 
impervious septum (Dural 0.020 in. in thick¬ 
ness), for which the average attenuation N, 
at medium and high frequencies, is governed 
by the weight law. 

The structure under test is clamped in a 
rigid steel frame of 17.5x17.5 in. inside dimen¬ 
sions and mounted on a steel plate fixed to a 
brick wall. A battery of nine loudspeakers 
mounted in a baffle and coupled to one (\pri¬ 
mary) side of the structure serves as a source 
of sound. The loudspeakers are backed by a 
box filled with rock wool which decreases sound 
transmission to the rear. 

On the other (secondary) side of the struc¬ 
ture under test there is located a long terminat¬ 
ing tube. The tube is fitted with four long 
wedges of Fiberglass The space between the 
wedge is filled with loosely packed cotton. This 
termination not only excludes undesired sound 
pickup from the outside but approximates 
closely an ideal termination of perfect absorp¬ 
tion. 

To measure the primary and secondary sound 
pressures, one microphone is located on each 
of the two sides of the panel under test. Their 
electric output can be connected at will to a 
high-gain amplifier and filter. The paper drive 
of a graphic level recorder is mechanically 
coupled to the oscillator driving the loudspeaker 
through a suitable power amplifier. By means 
of the recorder the outputs of the primary and 
secondary microphones are recorded as a func¬ 
tion of frequency. 

Figure 35 shows a cross section through the 
apparatus. Sound source, panel under test, 
absorptive termination, and microphones are 
clearly shown. Figure 36 shows the battery of 


c A pi’oduct of the Owens-Corning Fiberglas Corpora¬ 
tion. For dimensions of the wedges see Chapter 10, 
Section 10.9. 


speakers and the various frames for assembling 
structures of 3 in. total thickness. The Dural 
septum is shown clamped in place in the lower 
half of Figure 36. For details of construction. 



Figure 36. Primary and secondary sides of ap¬ 
paratus for measuring acoustic attenuation. 

electric equipment, experimental technique, 
reference 32 should be consulted. 

After the structure under test has been 
mounted in place and the absorptive termina¬ 
tion sealed to the secondary side, the sound 
source is energized by a sinusoidal signal over 
the frequency range of from 100 to 10,000 c. 
By means of the level recorder, graphic records 
of the outputs of the two microphones are made 
in succession. Typical records using a logarith- 














REDUCTION OF AMBIENT NOISE AND VIBRATION 


25 


mic frequency scale are shown in Figure 37 
for a Dural panel of 0.020-in. thickness. By 
point-by-point subtraction, taking due account 
of amplifier gains and microphone corrections, 
a curve of attenuation N versus frequency 


suitable averaging method, from experimental 
data obtained as described previously. By plani- 
metering the area under the two experimental 
curves obtained from the primary and sec¬ 
ondary microphone outputs, the average at- 



(similar to Figure 30) can be obtained. Clearly, 
an averaging method for evaluating graphs of 
such fluctuating character is required. 

To describe the attenuating properties of a 
structure in practical terms for the purposes 
at hand, it has been found useful to obtain 
the average attenuations N :i and N s at fre¬ 
quencies 3,000 and 5,000 c, respectively, by a 


tenuation values in a given frequency band can 
be obtained by subtraction of the two areas. 
The following octave bands have been success¬ 
fully used for this averaging procedure: 1,500 
to 3,000 and 3,000 to 6,000 c for N 3 ; 2,500 to 
5,000 and 5,000 to 10,000 c for N t . 

By entering the surface density o of the 
total structure (including Dural skin) into the 

































































































































































26 


SOUND CONTROL 


family of graphs for weight law given in Figure 
29, the attenuation values at 3,000 and 5,000 c 
predicted by the weight law can be obtained. 
The expressions n 3 and n 5 , the difference in 


attenuation at 3,000 and 5,000 c between the 
average measured attenuations and weight law, 
are a measure of the attenuation properties of 
the structure. In Figure 38, N a , N„ n 3 , and n- a 


Test 

Ho. 

Material 


Mfr. 

Typ« 

of 

Struc. 

MATERIAL 
Thick. Weight 
Inches Pounds 

Lbs/ft 2 

1 

3" AIR SPACE 



For Cal. 




2 

3" A IP SPACE, 

TRIM 






3 

3" A IP SPACE, 

SEPTTH 






k 

2" A IP SPACE, 

SEPTIJ4 






5 

2" A IP SPACE, 

SEPTUM, 


Ill 

1" air 




1* AIR SPACE; TRIM 



space m 



6 

AA 


O.C. 

ni 

1/2 

.052 

.024 

7 

AA 


SI 

hi 

1 

.104 

.049 

a 

AA 


• 

V 

1 

.104 

.049 

9 

DOT 


- 

in 

1/2 

.137 

.091 

10 

XKP7 


- 

in 

1 

.335 

.137 

11 

DOT 


IS 

V 

1 

.335 

.137 

12 

XMPT 


" 

III 

2 

.761 

.353 

13 

DOT 


•s 

V 

2 

.761 

.353 

1* 

RAY0R B 


DuP. 

II 

5/3 

.331 

.179 

15 

RAY0H B 


•• 

III 

5/3 

.331 

.179 

16 

RAY0H B 


* 

II 

1-1/4 

•732 

.363 

17 

RAYON B 


• 

III 

1-1/4 

.732 

.363 

13 

RAYOH B 


• 

V 

1-1/4 

.732 

.363 

19 

KAPOK NO. 1 


P.C. 

III 

1-3/4 

.706® 

.332® 

20 

KAPOK NO. 2 


• 

HI 

1-1/4 

. 626 * 

.2945 

21 

KAPOK NO. 2 


m 

III 

1-1/4 

.62o b 

.?94 b 

22 

KAPOK K3 



V 

2 

.330 

.414 

23 

COTTON FELT NO 

i. 1 


m® 

1/2 

.232 

.133 

2k 

COTTQH FELT NO 

. 3 


hi® 

1/2 

.377 

.177 

25 

M STONEFELT 


J.M. 

m 

1 

.53»* 

.251 

26 

H ST0NEFH.T 


m 

in 

1 

• 53k 

.251 

27 

5EFC 


O.H. 

V 

1 

.179 

.084 

23 

6dfc 


" 

V 

1 

.290 

.136 

29 

7IFG 



V 

1 

.376 

.177 

30 

51FC-1 


" 

V 

2 

• 3k7 

.163 

31 

D-k 


- 

V 


.172 

.081 

32 

G-k 



V 

1 

.273 

.131 

35 

WOOD COHV. 


w.c. 

III 

1 

.412 

.19k 

3k 

STPATOSEAL 


B.l. 

III 

1 

.763 f 

• 359* 

35 

HBSPOAS B 


O.C. 

III 

1 

.k!5 

.195 


Septum 

Weight 

Lbs/ft 2 

Trim 

Cloth 

Weight 

Lbs/ft 2 

Treat¬ 
ment Wt. 
Lbs/ft 2 

Total 

Struc. 

Weight 

Lbe/ft 2 

Total Attenuation 
3000 cpe 5000 cpe 

N* N5 

DB Better Than 
Weight Law 

n 3 “5 

0 

0 

0 

.273 

35.5 

kJ.O 

0 

0 

0 

.057 

•057 

• 330 

35-9 

40.9 

-1.5 

-0.8 

•061 

0 

.061 

•34 

42.7 

49.2 

5-5 

7-5 

.061 

0 

.061 

.334 

43.6 

49.8 

6.4 

8.1 

.061 

.057 

.118 

•391 

43.6 

50.1 

5.0 

7.0 

.061 

.057 

.142 

.kl5 

43.2 

53.1 

9.1 

Ik.5 

.061 

.057 

.167 

.440 

52.3 

65.O 

13.2 

20.9 

.061 

•057 

.167 

.440 

54.9 

63.0 

15.3 

23.9 

.061 

.057 

.209 

.482 

46.4 

5k.7 

6.0 

9.8 

.061 

.057 

.305 

.573 

50.3 

60.5 

3.3 

lk.O 

.061 

.057 

.305 

.573 

54.2 

67.1 

12.2 

20.6 

.061 

.057 

.476 

.749 

52.6 

64.7 

8.3 

15.9 

.061 

.057 

.476 

.749 

62.9 

75.1 

18.5 

26.5 

0 

.057 

.236 

.509 

41.4 

4^.7 

0.5 

2.3 

.061 

.057 

.297 

.570 

46.9 

56.4 

5.0 

10.0 

0 

.05-7 

.425 

.693 

k3.9 

56.3 

5.2 

3.1 

.061 

.057 

.4-36 

.759 

52.1 

61.3 

7.7 

12.9 

.061 

.057 

.486 

.759 

60.1 

72. k 

15.7 

23-5 

a 

.057 

• 339 

.662 

5&k 

70.0 

15.2 

22.3 

b 

.057 

.351 

.624 

56.6 

67.1 

13.9 

19.9 

.061 

.057 

.412° 

.685 

53.2 

72.8, 

14.7 

25. \ 

.061 

.057 

.532 

.805 

75.3 

95.? 

30.4 

k6.r 

.061 

0 

.19k 

.467 

47.6 

53. k 

7.k 

13.7 

.061 

0 

.233 

• 511 

43.0 

53-9 

7.1 

13.5 

.041* 

.057 

.3k9 

.622 

49.2 

53.6 

6.5 

n. k 

.061 

.057 

.369 

.642 

k9-9 

59. k 

7.0 

12.0 

.061 

.057 

.202 

• k75 

43.9 

59.9 

8.6 

15.1 

.061 

.057 

.25k 

.527 

51.7 

64.6 

10.5 

13.9 

.061 

.057 

.295 

.563 

53.6 

66.6 

11.7 

20.2 

.061 

.057 

.231 

.554 

55.3 

66.1 

14.1 

19.9 

.061 

.057 

.199 

.472 

43.7 

53.3 

3.4 

lk.O 

.061 

.057 

.2k9 

.522 

50.6 

60.7 

9.k 

15-0 

.061 

.057 

.312 

.585 

k7*.3 

56.0 

5.2 

9«k 

f 

.057 

.416 

.639 

64.4 

78.5 

20.9 

30.3 

.061 

.057 

.513 

.536 

57.6 

70.6 

15.5 

24.0 


_ p 

WDTBi All teats made with an .020" dural plate with a eurfaoe density of .273 lbs/ft • 

Asbestos Septum with surface density of .061 lbs/ft 2 was used for all teats except Test Bos. 1, 2, Ik, 16, 19, 20, 23, 3k. 
Arlato trim cloth with a surface density of .Q57 lbs/ft 2 vae used for all testa except Test Hoe. 1, 3, k , 23, 2k . 

Depth of structure 3" for all tests. 


a* Weight Includes attached paper septum. 

b* Weight Includes attached asbestos septum. 

c• Structure had attached septum plus a second adjacent septum. 

d. Attenuation found by extrapolating 6000-10,000 cpe portion 
• of 2500-10,000 cpe band. 

e. Ho trim cloth used. 
f» teglcekln septum. 


ABBRE7LAT~0NS USED FOR MANUFACTURER'S DISftHIATIOl 

O. C. - Owens Corning Flberglaa Corporation 
W.C. - Wood Conversion Ccmpany 

C.I. - Elavaty Insulation 

DuP. - E. I. duPont de Nemours ana Company 

P. C. - Philip Carey Manufacturing Company 
J.M. - Johns Manvllle 





-.020“ OURAL PANEL 
,0.273 LB/FT* 


CLOTH 


ACOUSTICAL 

MATERIAL 


STRUCTURE TYPE H 


STRUCTURE TYPE IH 


STRUCTURE TYPE I 


Figure 38. Numerical values of attenuation for various materials and structures 


























REDUCTION OF AMBIENT NOISE AND VIBRATION 


27 


are tabulated for a number of materials and 
structures. 

Figure 39 shows plots of N 3 versus N 5 and 
n s versus n 5 for the materials and structures 
tabulated in Figure 38. 

Figure 40 shows measured average values 
of N for two single impervious septa, in various 














































RELATIONSHIP BETWEEN THE OB BETTER THAN 

WEIGHT LAW ATTENUATION AT 5000 CPS AND AT 

3000 CPS FOR SOUNDPROOFING MATERIALS IN FIG 38 






























y 









































y 









































y 




















y 

y 





























































X 




















































































DECIBELS BETTER THAN WEIGHT LAW AT 5000 CPS 


are mounted in a reverberation chamber. After 
exciting the chamber with a sound source, the 
reverberation time is measured after the source 
has been turned off. From the reverberation 
time, the room volume, and the surface area 
of the samples, a can be computed. The absorp¬ 
tion coefficient so determined is physically 


i 
















7 



y 


















/ 



y 












/ 




























/ 


y 



















/ 



.020" OURAL PANEL AND 
- SOUNDPROOFING MATERIALS - 












/ 

y 

yy 




















y 



















, 












































r 




















/ 





















/ 




















/ 

















RELATIONSHIP BETWEEN THE ATTENUATION 

AT 3000 CPS AND AT 5000 CPS FOR 
SOUNDPROOFING MATERIALS IN FIG 38 




/ 








/ 









/ 











°0 10 20 30 40 50 60 70 80 90 100 110 

N 5 ATTENUATION AT 5000 CPS IN DECIBELS 


Figure 39. Relation between N and n for various materials and structures. 


frequency bands for certain positions of the 
primary and secondary microphones. The 
measured values fall below the values predicted 
by theory (weight law). The experimental 
apparatus was “calibrated” by applying the 
corrections from Figure 40 for the case of the 
0.020-in. Dural panel to adjust it to weight law. 

Measurement of Sound Absorption 

The sound-absorbing properties of a struc¬ 
ture have already been generally discussed in 
terms of an absorption coefficient a (see Sec¬ 
tion 2.2.1). The absorption coefficient a is, 
among other things, a function of the angle of 
incidence of the sound waves and has physical 
meaning as originally defined only in random 
sound fields. 5 - 19 The absorptive properties can 
also be described in terms of the acoustic 
impedance of the structure. Although the 
acoustic impedance of absorbent structures can 
be determined experimentally, 3 preference has 
been given here to the absorption coefficient. 

Two methods of measuring a are commonly 
in use. The first is the method approved by 
the Acoustical Materials Association 7 and is 
especially applicable to ordinary room acoustics. 
Large samples of the structure to be tested 


meaningful in the light of the original defini¬ 
tion, since it was determined under more or less 
random conditions of wave motion in the room. 

It was deemed advisable, in addition, to 
measure the absorptive properties of structures 
in terms of a quantity obtained from measure¬ 
ments in a tube. A sample of the structure 
(8x8 in.) is mounted at one end of a square 
steel tube. 18 A loudspeaker is mounted at the 
other end. When the sound source is energized 
by a sinusoidal voltage, a standing wave pat¬ 
tern is set up in the tube which can be explored 
by a movable microphone. From the ratio of 
the maximum and minimum microphone volt¬ 
ages, as the microphone is moved along the 
tube axis, a measure of the absorption coeffi¬ 
cient a for normal incidence can be obtained. 
The frequency range is limited, since the simple 
theory assumes, among other things, uniform 
pressure distribution in any cross section of 
the tube. 

Reference 18 contains a number of graphs 
of a vs frequency in the range from 100 
to 2,000 c obtained by the tube method and 
also a few data from chamber measurements. 
The data show that, at frequencies of 1,000 c 
or higher, the absorption coefficient is at least 



























































































































28 


SOUND CONTROL 


0.5 or greater for most materials. The contours 
of Figure 38 show that the sound-level reduc¬ 
tion is not likely to be improved significantly 
by even large errors in a, provided a is greater 
than 0.5. The values of a assigned to structures 
for computational purposes are a “weighted 


procedure and technique can be found in Re¬ 
port OSRD 1543. 1S _ 

To obtain the efficiency index E = ■\ / RT x /a m 
it is necessary to determine x by measuring 
R for various thicknesses T. The efficiency index 
E is usually taken for o m = 0.1 lb per sq ft. 




O 10 20 30 40 50 60 70 80 


COMPUTED ATTENUATION IN DB 


COMPUTED ATTENUATION IN DB 


Figure 40. Measured vs computed attenuation N for two panels (0.020 in. and 0.065 in. thickness). 


average” of the values obtained by the two 
methods (see Section 2.3). 

Measurement of Flow Resistance 

The apparatus used for the determination of 
the flow resistance R consists of the following 
components. 

1. A holder for the sample. 

2. A means of drawing air through the 
sample at a uniform rate. 

3. A gauge for measuring the pressure drop 
across the sample. 

4. A gauge for measuring the rate of volume 
flow of air through the sample. 

Figure 41 shows a picture of some of the 
components listed above. Circular samples of 
2%o in. in diameter are used. Details of the 


Army-Navy Specifications AN-S-32 a contain 
instructions for a battery of further tests of 
materials to test conformity with the general 
requirements for acoustic materials and struc¬ 
tures in Section 2.2.2. 


2 3 PREDICTIONS OF AMBIENT NOISE 
LEVELS IN AIRCRAFT 

From analysis of numerous data on ambient 
noise levels and spectra in multi-engine mili¬ 
tary aircraft accumulated during the years of 
1940 through 1944, an empirical method of 
predicting noise levels has been developed at 
the Electro-Acoustic Laboratory. A set of em¬ 
pirical graphs has been derived taking into 





































































































































PREDICTIONS OF AMBIENT NOISE LEVELS 


29 


account important variables such as engine 
power, propeller-tip speed, and amount and 
quality of acoustical treatment. It should be 
borne in mind that, through limitations of the 
original data from which the contours have 



Figure 41. Experimental arrangement for 
measuring the flow resistance of acoustic ma¬ 
terials. 


been obtained, these contours apply primarily 
to the multi-engine aircraft, now obsolete, 
which were in use in the closing years of World 
War II. They can be used as a rough guide 
only for aircraft of more modern design. In 
addition, the following assumptions were made: 

1. Two- or four-engine aircraft. 

2. Normal cruising power conditions. 

3. Three-blade propellers. 

4. Collector ring exhausts. 

5. Perfect seals around windows, hatches, 
etc. No secondary noise sources present in the 
cabin. 

6. Surface density of sound treatment (if 
any) less than 0.5 lb per sq ft. 

7. The reference point at which levels are 
computed is within 6 ft of the propeller plane, 


about 2 ft from the fuselage at the head level 
of a sitting man. 

In Figure 42 are shown typical spectra in a 
bomber, treated and untreated. For purposes 
of calculation, the frequency range has been 
divided in three regions, A, B, and C, as indi¬ 
cated. 

The level in region A can be predicted on the 
basis of engine power, propeller-tip velocity, 
minimum propeller-tip separation from the 
fuselage, and airspeed. Sound treatment of 
surface density less than 0.5 lb per sq ft does 


REGION A 



FREQUENCY OCTAVES 
(Cycles per second) 


Figure 42. Typical noise spectra in untreated 
(A) and treated (B) bomber. 


not reduce the levels in this region appreciably. 
Empirical graphs based on experimental data 
are given in Figures 4 and 10 of Report OSRD 
1543 18 for purposes of computation of sound 
levels. 

In regions B and C calculations consist in 
arriving at the slope of the spectrum from a 
computation of the ratio of total noise-absorb¬ 
ing area A to total noise-transmitting area T 
of the cabin. The total absorption A is simply 
the sum of the individual areas multiplied by 
the appropriate absorption coefficient a. The 
procedure outlined in Report OSRD 1543 18 
requires computation at 1,000 and 3,000 c. As 





















30 


SOUND CONTROL 


an average value of a for these frequencies 
the values 0.8 and 0.9 can be assumed. The 
total transmission T is the sum of the indi¬ 
vidual areas multiplied by the appropriate 
transmission coefficient t. The transmission 


bracketed by two computed spectra. Figure 43 
shows measured and computed spectra for four 
different aircraft and gives an estimate of the 
agreement of the predicted spectra with the 
computed spectra which can be expected. One 



Q 

Z « 
3 (E 
O ~ 
CO 


50 


40 


POWER 

CONDITION 

"bhp 

IAS 

mph 

IND 

ALT 

ft 

rpm 

Normal 

Cruising 

715 

220 

11,000 

2100 


i —r 


T—r 


OVER- <75 
ALL 


75 

150 


UJ 

H 

O u 
O 

<r c 
uj >> 
a. ■o 

oo ~ 

Q g 

z o 

J°' 
m M 


9 - 

o cc 

O “ 
co 


150 500 600 1200 2400 >4800 
300 600 1200 2400 4800 


130 

120 

110 

100 

90 

80 

70 

60 

50 

40 





J 1-1 1 





SK3MA 

lSTER 

C-54 














— 

S 

s 









\ > 

> 

s 

N 










\ 

\ 

> 









x" 

■% 

s 

s 



-Predicted 

-Measured 






v 

N 

V 

- r 

/ 

/ 

- POWER 
CONDITION 

thp 

IAS 

mph 

IND 

ALT 

ft 





/ 

/ 

rpm 


Normal 

Cruising 

550 

170 

3000 

2020 




FREQUENCY OCTAVES 
(Cycles per second) 


OVER- <75 75 150 300 600 1200 2400 >4800 

ALL 150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 


UJ 

H 

O o 
O ^ 

a: c 

ui >> 
a •o 

CDjV 

°§ 

Z o 

UJ 11 

> ® 
UJ > 
-J ® 

g «- 

Z <B 
3 (E 

o 

V) 


130 

120 

110 

100 

90 

80 

70 

60 

50 

40 


VEMTURA B-3 1 !- 


• Predicted 
-Measured 


POWER 

CONDITION 

hhp 

IAS 

mph 

IND 

ALT 

ft 

rpm 

Normal 

Cruising 

705 

150 

20,000 

1800 


OVER- <75 75 150 300 600 1200 2400 >4800 

ALL ISO 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 



OVER- <75 75 150 300 600 1200 240 0 4800 
Au 150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 


Figure 43. Measured and predicted noise spectra in airplanes. 


coefficient t can be arrived at for 1,000 and 
3,000 c from N ± and N 3 , taken from Figure 38 
with the help of equation (1). Figures 7 and 
8 in Report OSRD 1543 18 are empirical con¬ 
tours giving two values of spectrum slope for 
different values of A/T at 1,000 and 3,000 c. 
As a result, the spectrum to be expected is 


of the reasons for the large discrepancy in the 
case of the A-20-B airplane is its use of ejector 
exhausts. 

The computational procedure outlined above 
is useful as a guide either in the case of un¬ 
treated aircraft or in the case where the treat¬ 
ment is known and specified. Army-Navy Speci- 



















































































































































RECOMMENDATIONS FOR NOISE REDUCTION 


31 


fications AN-S-33 a require that the sound level 
in the frequency band 1,200 to 2,400 c be less 
than 80 db at the head position of the crew 
members. If this requirement is to be attained 
in a given airplane, it becomes necessary to 
attempt the design of the optimum sound treat¬ 
ment, since weight is at a premium. The mini¬ 
mum surface density a and the minimum 
efficiency index E necessary to meet the specifi¬ 
cations for a given airplane must be found. 
Procedures for accomplishing this are con¬ 
tained in Part 3 of Report OSRD 1543. 18 By 
successive approximations a treatment of mini¬ 
mum total added weight can be arrived at. 


2 4 RECOMMENDATIONS FOR NOISE 
REDUCTION AND CONCLUSIONS 

The discussion in the preceding sections will 
now be made the basis for a brief list of recom¬ 
mendations for installing optimum sound treat¬ 
ment in certain combat vehicles. Examples have 
been chosen in accordance with their general 
importance. At the same time, their choice is 
conditioned by the assignment of particular 
problems to the Electro-Acoustic Laboratory 
for solution. Judicious application is possible 
and desirable to other cases not listed. 


2 - 41 Aircraft 

1. Noise reduction of the medium- and high- 
frequency components of the noise is necessary 
to avoid impairment of voice communications 
and reduce annoyance. 

2. The effectiveness of sound treatment in 
an airplane cabin is determined primarily by 
the attenuation of transmitted sound of the 
structure, the external sound sources, and the 
absorption of sound by the structure. 

3. The effectiveness of a sound treatment 
is, at best, only as good as the weakest parts 
of the cabin walls transmitting sound from the 
outside. Hence windows, floor, and untreated 
portions of the Dural skin must be sealed and 
sufficiently heavy so that the sound transmitted 
by these areas will not contribute significantly 
to the noise level in the cabin. 


4. For minimum noise levels, blowers, 
auxiliary power plants, and other internal noise 
sources must be effectively silenced. 

5. In multi-engine airplanes of the type used 
in the closing years of World War II, it is 
possible to reduce the noise levels to meet 
Army-Navy Specifications AN-S-33 a , if suffi¬ 
cient weight of treatment is added. 

6. For a given airplane, there is a minimum 
surface density of treatment to meet the speci¬ 
fications if a material with an efficiency index 
greater than a minimum value is used. 

7. The minimum weight of added treatment 
can only be realized if points (3) and (4) 
are observed. 

8. Optimum structures simple to install and 
to fabricate are given in the Army-Navy Speci¬ 
fications AN-S-33 a . 

9. Requirements for installation and mount¬ 
ing of the treatment are contained in Army- 
Navy Specifications AN-S-32 a . 

10. The acoustical requirements should be 
made part of the basic design considerations of 
an airplane. 

2 ' 4 ' 2 Combat Information Centers Aboard 

Ship 

1. Noise reduction of the medium- and high- 
frequency components of the noise is necessary 
to avoid impairment of voice communication. 

2. The effectiveness of sound treatment in a 
closed compartment on shipboard depends 
primarily on the absorption of sound by the 
structure. Owing to the presence of heavy bulk¬ 
heads and decks, sound transmission is usually 
inconsequential. 

3. Internal noise sources: blowers, type¬ 
writers, 26 etc., must be effectively silenced. 

4. A blanket of certain absorbing materi¬ 
als, 23 2 in. thick, spaced 1 in. from the bulkhead 
or ceiling, is recommended. 

5. The acoustic material should have a flow 
resistance between 20 and 30 g per sq cm 
per sec per in. thickness. 

6. Open metal mesh covers with a large ratio 
of area of holes to total area or trim cloths 
(R less than 20 g per sq cm per sec) are 
suitable to cover the material. 



32 


SOUND CONTROL 


2 ' 4 ' 3 Fortifications 

1. The reduction of the frequencies of shock 
and explosion noises, which comprise mainly 
the low end of the audible frequency band, is 
desirable to avoid detrimental effects on combat 
personnel 27 and voice communications. 

2. To provide a structure with high absorp¬ 
tion coefficient down to low frequencies, the 
thickness must be large. Four inches is recom¬ 
mended as the minimum thickness. 

3. As large an area as possible should be 
treated. 

4. Ventilation ducts, gun embrasures, etc., 
should be lined with acoustic material, and 
acoustic leaks to the outside should be mini¬ 
mized. 

5. Aural protective devices should be worn 
by the personnel (see Section 2.6). 

6. In addition to acoustical treatment, insu¬ 
lation against mechanical shock is desirable. 


2 5 THE EFFECT OF NOISE ON PSYCHO¬ 
MOTOR EFFICIENCY 

One of the first military problems undertaken 
by the Psycho-Acoustic Laboratory was to dis¬ 
cover the effects of intense noises on psycho¬ 
motor efficiency. It was believed that by measur¬ 
ing the various aspects of a subject’s mental, 
motor, and physiological behavior under the 
stress of noise and vibration, one should be 
able to determine the practical degree to which 
this environmental stress should be eliminated 
from combat situations. 

Apparent from the beginning was the ex¬ 
treme difficulty of evaluating in a quantitative 
manner the role of noise in the great complex 
of variables and conditions affecting all human 
performance. Acceptable standardized tests of 
even so obvious a phenomenon as human fatigue 
were largely nonexistent, and many efforts to 
probe the psychophysiological effects of in¬ 
ternal and external stresses have been defeated 
by the overwhelming complexity of the factors 
conditioning human activities. It was decided, 
therefore, that the first effort should go toward 
the design and development of psychological 
and motor tests capable of sampling and 


evaluating a wide range of tasks which might 
be required under stress of noise. 

The noise stress which was used for most 
of the tests simulated the noise encountered in 
the cabin of a typical bombing plane. A voltage 
of uniform spectrum can be produced by ioniza¬ 
tion in a gaseous tube, and a low-frequency 
voltage rich in harmonics can be generated by 
a relaxation oscillator. When these two voltages 
are properly mixed, amplified, filtered, and 
equalized, and led to a battery of loudspeakers, 
the resulting sound has a spectrum similar to 
that of an airplane noise. 

The spectra of the two simulated airplane 
noises used in the following experiments are 



OVER- <75 75 150 300 600 1200 2400 >4800 
ALL 150 300 600 1200 2400 4800 

FREQUENCY OCTAVES 
(Cycles per second) 

Figure 44. Sound level per octave band for the 
two synthetic airplane noises used in the tests 
of psychomotor efficiency. The spectra were de¬ 
signed to simulate the noise in acoustically un¬ 
treated and acoustically treated planes. See 
Figure 42. 

shown in Figure 44. These spectra were de¬ 
signed to simulate the typical noise condi¬ 
tions in acoustically untreated and acoustically 
treated planes. (See Figure 42.) 

The battery of psychomotor, physiological, 
and psychological tests included measurements 
of the near point of vision, speed of accommo¬ 
dation, speed of eye movement, visual acuity, 
muscular tension, heart rate, finger tremor, 
blood pressure, marksmanship, card sorting, 
span of apprehension, paper form board tests, 
steadiness tests, tapping board and standard 



























NOISE AND PSYCHOMOTOR EFFICIENCY 


33 


pursuit rotors for testing motor coordination, 
coordinated serial pursuit meters which par¬ 
tially simulated the requirements of instrument 
flying, tests of coordinated serial reaction time, 
audiometric measurements of hearing loss, vi¬ 
bration tests, tests of serial disjunctive reac¬ 
tion time, coding tests, judgments of distance, 
etc. With this extensive battery it was possible 
to take a very thorough sample of the subject’s 
behavior both in the presence of noise and in 
the quiet. 

The results of a preliminary series of tests in 
which subjects were exposed for three hours to 
a noise (approximately 115 db) demonstrated 
that acoustic stress is detrimental to certain 
aspects of performance but leaves other aspects 
unimpaired. 6 Since the effects of the noise on 
psychomotor efficiency were in most cases very 
small, and because there was some evidence 
for a greater loss in proficiency at the end than 
at the beginning of the three-hour experiments, 
it was necessary to test the implication that 
longer exposure to the noise would exhibit 
these effects still more conspicuously. Conse¬ 
quently, a second series of experiments was 
designed in which the effects upon psychomotor 
efficiency of continuous seven-hour periods of 
work in an airplane noise of 115 db could be 
evaluated by comparison with the effects of 
similar periods of work in an airplane noise of 
only 90 db. 9 The intensity of 90 db is just about 
sufficient to mask the casual noises of the 
laboratory and to discourage conversation 
among the subjects. 

Five carefully selected subjects were em¬ 
ployed for a period of two months in these 
experiments. During the first few weeks, the 
subjects were tested in miscellaneous ways and 
allowed to practice the performance tests in 
order to reach a plateau in learning from which 
later improvement due to practice would be 
minimized. Following this preliminary period, 
the subjects were tested in the main series of 
experiments requiring four consecutive days in 
each of four consecutive weeks. In this series, 
the conditions of “noise” (115 db) and “quiet” 
(90 db) were distributed over the sixteen ex¬ 
perimental days in such a way as best to 
counterbalance such systematic variables as 
learning, boredom, etc. On the other days of 


the week, tests involving procedures not com¬ 
patible with this counterbalanced order were 
applied. Most of the tests were so intensive, 
the control so adequate, and the counter¬ 
balanced order so effective that differences in 
performance of the order of 1 per cent could 
be detected with high reliability. 

In tests of this nature, the importance of 
maintaining a stable level of motivation for 
the subject cannot be overemphasized. Perhaps 
the influence on efficiency of this factor of 
motivation may best be demonstrated by an 
example from the results during the practice 
period for the coordinated serial reaction-time 
test. On one of the practice days, the experi¬ 
menter noticed that the scores during the 
eighth period were much better than for the 
preceding periods of that day. The subject was 
asked later if he could account for this fact. 
He reported that by self-instruction, his gen¬ 
eral level of muscular tension had been greatly 
increased. The experimenter could see that the 
subject’s grip on the chair with his free hand 
was tightened, and his facial expression re¬ 
vealed considerable muscular tension. The sub¬ 
ject reported that he could not possibly main¬ 
tain such a level of activity for an extended 
period, but that he had tried for the one block 
of trials to push himself to extreme effort. 

The subject was, therefore, asked each day 
for four consecutive days at the beginning of 
the eighth period to duplicate as well as he could 
the exact conditions of tension and effort ex¬ 
perienced on the first day, and to run through 
the sequence as rapidly as possible. As the 
curve of Figure 45 shows, this increased moti¬ 
vation resulted in a striking improvement in 
the average performance of the eighth period. 
The average for the succeeding blocks, how¬ 
ever, returns to the subject’s typical level of 
performance. 

Since there is no way to control completely 
all fluctuations in effort, any experiment in 
which one hopes to minimize the effect of 
changing motivational conditions must rely on 
a large number of scores taken under the same 
conditions. If the fluctuations are not system¬ 
atic with respect to changes in noise level 
they can be expected to cancel out, but only 
if a large number of scores are taken. 




34 


SOUND CONTROL 


The results of these experiments agree with 
common sense. Although, as a subjective ex¬ 
perience, noise is disagreeable and tiring, most 
types of mental, motor, and physiological activi¬ 
ties are affected very little by noise as such. 
Just as when a man steps from an airplane 
after a long flight at moderate altitude, he is 



Figure 45. Showing the change in performance 
introduced by intense effort during one of a 
series of 12 trials. 


able to walk, talk, write, think, etc., in about 
the same way as before the take-off, so in the 
laboratory no dramatic change in behavior 
follows exposure to airplane noise. Confirming 
evidence for this general conclusion was ob¬ 
tained at Stevens Institute of Technology in an 
investigation of sound as a military weapon. 13 
It is found, in general, that communication is 
difficult or impossible, hearing losses result, 
and subjective experiences of fatigue and 
annoyance are reported, but no conclusive evi¬ 
dence appeared to confirm the notion that 
intense noise interferes with the performance 
of motor tasks. 


251 Tests Showing Detrimental Effects 
of Noise 

Temporary Hearing Loss 

Long exposure to airplane noise, similar to 
that encountered in a bombing plane having 
little or no acoustical treatment, results in 
decreased auditory sensitivity (stimulation 


deafness). These hearing losses are greatest in 
the region from 3,000 to 5,000 c, but after eight 
hours of continuous exposure, losses as great 
as 30 db occur at frequencies as low as 250 c. 
In all cases studied these impairments proved 
temporary, but the fact of their occurrence is 
proof of the stress imposed on the auditory 
organ. As the duration of the exposure is in¬ 
creased, the stress, as manifested in hearing 
loss, is increased and the time required for re¬ 
covery is prolonged. 

Stimulation deafness is related to three 
obvious aspects of exposure to complex noises. 
These are (1) the overall intensity of noise, 
(2) the shape of the noise spectrum, and (3) 
the duration of exposure to noise. 

The amount of change in the threshold is 
directly related to the intensity of the noise. 
The most severe impairment results at the 
higher frequencies. This becomes even more 
striking upon consideration of the fact that the 
noise spectra used for these exposures have 
by far the greater part of their energy con¬ 
fined to the lower frequencies. The low fre¬ 
quencies are not discernibly impaired as a 
result of stimulation for 30 minutes by intensi¬ 
ties below 120 db. 

The effect of the shape of the spectrum on 
the hearing loss is illustrated in Figure 46. In 



FREQUENCY IN CYCLES PER SECOND 

Figure 46. Illustrating hearing losses resulting 
from 30 min exposure to 110 db of two synthetic 
airplane noises. 

this figure, the noise spectrum used to obtain 
the hearing loss indicated by the closed circle 
was similar to that encountered in a bombing 
plane without acoustical treatment. The hear¬ 
ing loss indicated by the open circle w T as ob¬ 
tained using a spectrum of noise similar to that 









NOISE AND PSYCHOMOTOR EFFICIENCY 


35 


encountered in a bombing plane with acoustical 
treatment (see Figure 44). Acoustical treat¬ 
ment attenuates the high-frequency compo¬ 
nents of the noise, and the effect of this 
difference in high-frequency energy is apparent 
from the two curves. 

The effects of the duration of exposure to 
the noise are shown in Figure 47 and Figure 
48. Figure 47 shows the effect of short exposure 



Figure 47. Showing the relation between the 
severity of hearing loss and the duration of ex¬ 
posure to 105 db of white noise. 


to a noise having uniform distribution of 
energy throughout the frequency range and 
Figure 48 shows the effect of long duration 
of exposure to a spectrum similar to that en- 



Figure 48. Illustrating hearing loss caused by 
long exposures to 115 db of noise simulating a 
bomber without acoustic treatment. With long 
exposures the effect spreads to the lower fre¬ 
quencies. 

countered in an acoustically untreated bomber. 
In both cases it can be seen that the amount 
of hearing loss increases as the length of 


exposure is increased, and that with the longest 
exposures, the hearing loss spreads to the lower 
frequencies. 

Audiograms taken during the course of re¬ 
covery from exposure to airplane noise one-half 
hour at 110 db are shown in Figure 49. In 



Figure 49. The course of recovery from one- 
half hour exposure to 110 db of noise from a 
plane without acoustic treatment. 


general, the recovery is most rapid immediately 
after exposure. The higher frequencies suffer 
greater damage and recover more slowly than 
the low frequencies. 

Subjective Experiences 
Associated with Noise 

The subjects were required to fill out ques¬ 
tionnaires daily regarding their impressions. 
Uniformly, they expressed decided preference 
for the days on which they worked in quiet 
(90 db). They showed a tendency to report 
greater feeling of fatigue at the end of the 
experiments in noise, but this report was not 
consistent. After every experiment in noise, 
all subjects had reported ringing in the ears 
(tinnitus), but in only one instance did a sub¬ 
ject report tinnitus on a quiet day. 

The subjects also reported that some noises 
were more annoying than others. For example, 
it was found that the noise from a bomber 
having acoustical treatment was much less 
annoying than the noise from an untreated 
plane. In order to make these two noises equiva¬ 
lent in annoyance value, it is necessary to make 
the noise of the treated bomber 10 db more 
intense than the noise from the untreated 
bomber. 















36 


SOUND CONTROL 


In order to investigate the factors underlying 
these differences in annoyance, an analysis was 
made of the relative annoyance produced by 
various frequency bands of noise. 20 A “white” 
or uniform noise containing all frequencies 
from 40 to 9,000 c at approximately equal in¬ 
tensity was divided by an appropriate filter 
system into twelve bands. The frequency band 
from 1,900 to 2,450 c was used as the standard 
noise band and subjects were required to equate 
each of the other eleven bands to this standard 
band with respect to (1) loudness and (2) 
annoyance. The results of this procedure are 
shown in Figure 50. In this figure, loudness is 






_ 

L^l 

_ 







i! 







H 

-..J 


[—— 

1 SOUND PRESSURE LEVEL 

(STANDARD BAND *94 DE 

OF 1 










1 | 












r 



—• i 1 • ' 




| SOUND PRESSURE LEVEL OF 1 
(STANDARD BAND = 64 DB | 



Hr 


NOMINAL CUTOFF FREQUENCIES OF THE BANDS 


Figure 50. Difference between loudness and 
annoyance judgments. Loudness is plotted as a 
straight horizontal line for both intensity levels. 
The dashed line is drawn through the average 
difference in decibels between the loudness and 
annoyance judgments for five subjects. 


plotted as a straight line for each of four 
intensity levels at which judgments were made, 
and the difference in decibels between the loud¬ 
ness and annoyance judgments for each of five 
subjects is shown by separate points for each 
band. In general, the results show that loudness 
and annoyance are separate and distinct dis- 
criminable characteristics of sound. Among 
bands adjusted to be equal in loudness, those 
bands of noise composed of high frequencies 
are definitely more annoying than the standard 
band. 

The relatively greater annoyance of the high 
frequencies is also true for tonal signals. 28 In 
general, studies of annoyance have shown it to 
be directly related to three important variables, 
(1) the loudness of the noise, (2) its pitch, 
and (3) modulation of the pitch and loudness. 
The most annoying sounds are those whose 
energy is concentrated in the high frequencies 


and whose pitch or loudness is changing 
randomly and unpredictably. 

Intelligibility of Speech 

The most serious hazard of airplane noise 
arises from its interference with communica¬ 
tions. Intense ambient noises in airplanes inter¬ 
fere not only with direct person-to-person 
communication but also are picked up in micro¬ 
phones and leak in around earphone cushions. 
Because of its importance, the investigation 
of noise interference with communication sys¬ 
tems became a major research problem, and 
the results are treated in considerable detail 
in the following chapters. 


2 5 2 Tests Showing Indeterminate Results 

Certain tests gave inconclusive results, owing 
to large individual differences among the 
subjects, to wide variability among the results 
from a single subject, or to certain difficulties 
in the experimental procedures. 

Coordinated Serial Reaction-Time 

The coordinated serial reaction-time ap¬ 
paratus, shown in Figure 51, was developed to 
measure in an objective way the degree of 
proficiency with which a subject can coordinate 
the eyes, arms, and legs. In this test the subject 
manipulates airplane controls to direct a rec¬ 
tangular beam of light at a series of targets 
reached by following definite paths. The rudder 
bar is coupled to the lamp in such a way that 
it moves the beam of light horizontally from 
right to left. Forward and backward move¬ 
ments of the stick raise and lower the beam in 
the vertical plane, and right and left movements 
of the stick rotate the beam through an angle. 

Both the total time required for a set of 
reactions and the errors (false moves) made 
as departures from the correct path were re¬ 
corded. Each of the five subjects made 20,400 
reactions in noise and the same number in 
quiet. The reaction in noise was slower by 5.4 
per cent. Also, the number of errors committed 
in noise was greater by exactly the same pro¬ 
portion, 5.4 per cent. 

Of all the motor-coordination tests employed, 
























NOISE AND PSYCHOMOTOR EFFICIENCY 


37 


this was the only one which showed a decrement 
in performance in the presence of noise. Other 
tests which appear to be closely related to this 
serial reaction-time test showed no effect, as 
will be seen below. A consideration of several 


Subjects are apparently able to relax as com¬ 
pletely in noise as in quiet, provided they try 
to relax, but when records are made at a time 
when the subjects are busy at another task, 
there is evidence of increased tension due to 



Figure 51. Coordinated serial reaction-time apparatus. 


possible explanations of this apparent paradox 
led to the belief that in the less intense noise 
the subjects made use of certain acoustic cues 
provided by the “click” of timing relays which 
controlled the apparatus. This possible artifact 
was not experimentally tested because of the 
press of time, and consequently the slight 
decrement obtained by this apparatus cannot 
be considered valid until further tests are made. 

Muscular Tension 

There is no doubt that noise causes in some 
subjects an increase in muscular tonus, but 
attempts to measure this effect by recording 
electric potentials in various muscles were often 
frustrated by artifacts of gross muscular move¬ 
ments. Voltages from the arms and legs were 
recorded under various conditions or activities. 


noise. But as already indicated, artifacts of 
movement too often obscure the records. 

Metabolism 

Since the variation in metabolic rate from 
day to day and even during a single day is 
so large, it was not feasible to determine the 
effects upon metabolism of long exposures to 
intense noise. Shorter exposures repeated sev¬ 
eral times during the day indicate that for some 
subjects there is an increase in metabolic rate 
as a function of noise. In other subjects no 
consistent effects could be noted. 

Breathing 

Breathing was recorded in the process of 
measuring metabolism and also under other 
experimental conditions. Noise definitely causes 








38 


SOUND CONTROL 


some subjects to breathe more rapidly and less 
deeply, but here again variability and indi¬ 
vidual differences forbid generalizations. 

Speed of Accommodation 

The speed with which the focus of the eye 
can be changed from a near to a distant object, 
and vice versa, is apparently reduced by long 
exposure to noise. This effect was present in 
many of the experiments, but individual dif¬ 
ferences do not permit a simple statement of 
the magnitude of the reduction. 

Saccadic Eye Movements 

The speed with which the visual line of re¬ 
gard can be shifted through an angle of 37 
degrees was measured for four subjects for 
more than 5,000 eye movements, but in only 
one subject was the speed of movement slower 
in noise than in quiet. 

Body Sway 

The ability of a subject to stand erect without 
swaying was measured by an ataxiameter. 
Although this performance was not tested in¬ 
tensively, preliminary results suggested that 
there exists no striking relation between steadi¬ 
ness and noise. 

Hand Steadiness 

The subjects showed wide individual differ¬ 
ences in their ability to hold a small stylus in 
the center of a small hole (3-mm diameter) 
without touching the walls. The effect of noise 
was to produce a slight but statistically unre¬ 
liable increase in steadiness. It is possible to 
attribute this improved performance to the 
ability of the airplane noise to mask other 
distracting stimuli and thereby “insulate” the 
subject from his environment. This “insulat¬ 
ing” effect was also reported by the subjects 
who participated in the experiments on marks¬ 
manship. The results of the studies on 
marksmanship indicated that noise of the 
temporally continuous character employed 
tends to increase the accuracy of offhand shoot¬ 
ing. 

Reversible Perspective 

When a person views an outline drawing of 


a figure such as a cube or a staircase he observes 
periodic shifts in perspective. The cube appears 
sometimes as seen from above and sometimes 
as seen from below. The subjects were asked 
to fixate a reversible figure and to press a key 
at the occurrence of each involuntary reversal. 
If there is any relation between the instability 
of this type of perception and noise it was in 
our experiment obscured by variability. 

Dark Adaptation 

The threshold of visual illumination was 
measured at the beginning and at the end of 
seven-hour periods of exposure to noise. These 
thresholds were actually about 0.05 log units 
higher than after a similar period in quiet, but 
this difference may not be attributable to the 
effect of noise on the subjects because a similar 
difference occurred at the very beginning of 
the seven-hour period. Additional research is 
needed to discover the reason for these results. 
Also there appeared a slight but insignificant 
effect on the visual threshold when the noise 
was turned on and off at approximately ten- 
minute intervals. 


2 5-3 Tests on Which Noise Had No Effect 

The following tests are those on which the 
subjects made the same score in noise as in 
quiet. The results of these tests are not to be 
considered negative in the sense of being 
inconclusive. They are positive in the sense that 
they prove that, under long exposures to intense 
sound, subjects are able to maintain their levels 
of performance on a wide variety of tasks. 

Coordinated Serial Pursuit 

The coordinated serial pursuit meter was 
developed as a task which partially simulates 
the requirements of instrument flying. The 
subject is required to adjust by means of air¬ 
plane controls the position of the spot on a 
cathode-ray oscillograph and the position of the 
pointer on a direction meter in such a way as 
to compensate for deflections introduced by 
external circumstances. A view of this appa¬ 
ratus is shown in Figure 52. 

A graphic record of each of the three dimen- 



NOISE AND PSYCHOMOTOR EFFICIENCY 


39 


sions of movement (forward-backward and 
lateral on the oscilloscope and direction on a 
separate meter) is written on three Esterline- 
Angus recording galvanometers. In this way, 
a record is obtained which reveals not only the 


hand or foot, depending on which of four lights 
was illuminated in front of him. Each subject 
made 24,480 reactions in noise and the same 
number in quiet. Had there been a difference 
between noise and quiet of 0.6 per cent it would 



Figure 52. Coordinated serial pursuit meter. 


number or extent of the subject’s failures, but 
also the nature of these errors. 

Subjects were able to maintain their per¬ 
formance on this test unimpaired in airplane 
noise. There was some indication in the verbal 
reports of the subjects that they maintain their 
performance by dint of greater effort. At least, 
they were aware of a tendency to compensate 
for the stress of the noise. If it was in fact 
a matter of compensation, it is possible that, if 
the pursuit meter were speeded up sufficiently, 
a significant difference between the perform¬ 
ance in quiet and noise might appear. 

Serial Disjunctive Reaction-Time 

Quite unimpaired in noise was the ability of 
the subject to press a key with the appropriate 


have been statistically significant. The actual 
difference was only 0.1 per cent. 

Fast-Speed Pursuit Rotor 

Also unimpaired was the ability of a subject 
to follow with a stylus a small disk near the 
edge of a phonograph table revolving 99 rpm. 

Card Sorting 

It was demonstrated early in the experiments 
that ability to sort 12 kinds of cards into 12 
compartments is maintained as well in noise 
as in quiet. As soon as this was discovered the 
test was dropped from the procedure. 

Coding Test 

The subjects were able to translate written 





40 


SOUND CONTROL 


material into code, letter by letter, as rapidly 
in noise as in quiet. 

Judgment of Distance 

Noise had no effect on monocular judgments 
of distance as measured by the ability of the 
subjects to adjust the distance of a movable 
wire until it was as far away as a standard 
wire. 

Conclusion 

Consideration of these results leads to the 
conclusion that airplane noise, apart from the 
other stresses to which an aviator is subjected, 
has at worst only a slightly detrimental effect 
upon functions involving motor coordination, 
reaction-time, sensory perceptions, and certain 
mental functions. In most instances it can be 
positively demonstrated that noise has no effect, 
even after exposures lasting seven hours. Func¬ 
tions such as breathing, metabolism, and muscle 
tension are sometimes affected by noise, but 
what the effect will be seems to depend upon 
the kind of individual tested, and simple gen¬ 
eralizations are not possible. 

Nevertheless, all subjects seem to prefer not 
to work in an intense noise, and after a day 
under acoustic stress, they tend to report a 
subjective feeling of being more tired and 
irritable. They have a ringing in the ears and 
a temporary hearing loss to testify to the 
severity of the conditions under which they and 
the pilots of aircraft labor. 

The most severe effects of noise are upon 
the ear itself. Speech communications are im¬ 
paired and temporary losses in hearing are 
produced. The problem of stimulation deafness 
was later assumed under the Committee for 
Medical Research, 16 - 17 while the studies of 
communication in noise were continued by 
NDRC Section 17.3. 


254 Effect of Vibration on Visual Acuity 

Ten subjects were tested for visual acuity 
under two conditions. Under one condition the 
subjects were seated on a stationary chair. 
Under the other condition the chair vibrated 
at a rate of 2,300 rpm. Variable-speed motors, 


driving eccentric loads, were used to generate 
vibrations similar to those encountered in air¬ 
planes. Visual acuity was measured by means 
of a grid of parallel lines whose separation 
and whose angle with the horizontal could be 
continuously varied. 

In every subject vibration produced a con¬ 
siderable reduction in visual acuity. The aver¬ 
age reduction for all subjects was 25 per cent. 
The reduction for horizontal lines was 5 per 
cent greater than for vertical lines. The reduc¬ 
tion seems to depend upon the effective vibra¬ 
tion at the subject’s head. For a fixed vibration 
of the chair, this effective amplitude at the 
head appears to depend upon the body-build 
of the individual. Measurements showed an 
amplitude of vibration at the head equal to 
approximately 0.001 in. 

Further study of the effects of vibration on 
psychomotor efficiency were planned, but a 
growing preoccupation with communication 
problems made it necessary to abandon the 
vibration research. 


26 AURAL PROTECTIVE DEVICES: 

EAR WARDENS 

Although most ears recover completely with¬ 
in a few hours from the impairment that fol¬ 
lows the usual airplane mission or a day in 
a noisy factory, there is considerable indirect 
evidence that permanent hearing losses, par¬ 
ticularly for high tones, may follow repeated 
exposures to acoustic stress. Protection of the 
ear against these effects can be accomplished 
by isolating the sources of the noise, by acousti¬ 
cal treatment of the surroundings, or by 
stopping the ears themselves against the sound. 
This last method is the most applicable in the 
usual military situation, and for this purpose 
the development of a satisfactory earplug was 
undertaken. 

The earplug was designed to meet three re¬ 
quirements: acoustic insulation, comfort, and 
distortionless transmission of speech frequen¬ 
cies. Comfort can be ranked as a consideration 
equal in importance to acoustic insulation. The 
material must be soft and compliant enough to 
shape itself to the contours of the ear canal. 



AURAL PROTECTIVE DEVICES: EAR WARDENS 


41 


Also, an earplug intended for military use must 
not impede communication under the conditions 
in which the earplug must be worn. An earplug 
that reduces the transmission of noise also re¬ 
duces transmission of speech sounds. Hence in 
situations where loud noises make the use of 
earplugs desirable, the earplug selected should 
attenuate all the frequencies of speech approxi¬ 
mately the same extent. This insures against 
distortion of the speech by the protecting device 
while, at the same time, it reduces the whole 
complex of speech and noise to a level to which 
the wearer can more comfortably listen. At this 
more comfortable level, a given speech-to-noise 
ratio can result in improved communication. 

These basic requirements were recognized 
when the initial NDRC project was undertaken 
in November 1940 in the Department of 
Physics, University of California at Los An¬ 
geles [UCLA]. 11 The program of this project 
resulted in the development of the Type V-29 
ear defender. The principles established by this 
work guided the subsquent development at the 
Psycho-Acoustic Laboratory, where the NDRC 
Project was continued. Of the resulting models, 
the ear warden Type V-51R semed to offer the 
best combination of desirable properties and 
this model was consequently recommended for 
Service use. 


261 Design 

The body of the V-51R ear warden is elliptical 
in cross section rather than round, since this 
shape has been found to approximate more 
closely the shape of the average ear canal. 
Adapting the earplug to the shape of the ear 
canal aids in securing for the wearer greater 
comfort and better protection. A longitudinal 
section of the earplug is shown in Figure 53. 

The bell-shaped flange on the forward end of 
the ear warden aids in establishing a good seal. 
The septum is a dome-shaped structure, convex 
outward, situated at the beginning of the stem 
which holds the bell-shaped flange. This stem is 
hollow for greater flexibility. The safety tab 
perpendicular to the axis of the main body aids 
in sealing the earplug in the ear canal and pre¬ 
vents the earplug from being inserted too far. 


The removal pad makes it easy to withdraw the 
earplug. 

The theory of earplugs has not been fully 
formulated. This is explained in part by the 
extreme complexity of the acoustic system 



Figure 53. Cross section diagram of the V-51R 

ear warden. 

which results from the introduction of an ear¬ 
plug into the ear canal, and in part by the fact 
that the experimental determination of the 
various acoustic quantities requires elaborate 
experimental apparatus. In the V-51R ear war¬ 
den the bell-shaped flange and the thin, com¬ 
pliant body wall are designed to reduce leakage 
to a practical minimum. Shaping the septum 
and related parts in such a manner that a high 
stiffness factor obtains minimizes transmission 
through the earplug. The small mass of the ear 
warden, together with the fact that the body 
wall offers a large frictional area of contact, 
tends to obviate any tendency of the ear warden 
to oscillate as a whole. 

From a theoretical point of view, any of the 
variables—mass, stiffness, or frictional resist¬ 
ance—may be used as a means of preventing 
the transmission of sound through a device, and 
the particular combination of these properties 
determines the frequency characteristic of the 
transmission loss. In particular, it is important 
to emphasize that both stiffness and friction can 
serve as the principal impedance and to mini¬ 
mize the importance sometimes attached to 
mass. 

The earlier work at UCLA produced several 
designs involving mass as the principal im- 
















42 


SOUND CONTROL 


pedance. In order to secure comfort for the 
wearer, the mass was incorporated in a com¬ 
pliant structure, and, as might be expected, 
some of the devices behaved as tuned circuits, 
with a consequent loss of insulation. The V-51R 
ear warden, like the V-29 which preceded it, was 
designed to present mainly a stiffness im¬ 
pedance to the incoming sound. In such a device, 
adequate impedance to frequencies between 100 
and 1,000 c can be had from stiffness and fric¬ 
tion, provided the resonant frequency of the 
earplug in its suspension in the ear canal is 
high, and provided the impedance at the res¬ 
onant frequency is largely dissipative. 

Use of wardens in aircraft suggests a special 
problem of design. There is evidence that equal¬ 
ization of the pressure in the ear canal and in 
the middle ear is not readily accomplished by 
some people if the changes in altitude are very 
large or very rapid. 22 In a minority of ears 
inflammation of the ear canal developed. A 
special slow leak in the septum of the earplug 
is a possible answer to this difficulty. 


2 62 Material 

The ideal material for the manufacture of 
earplugs would be at once malleable and re¬ 
silient. Its physical characteristics would not 
be affected by aging, by exposure to temper¬ 
ature extremes, or by the action of earwax, 
antiseptics, and cleansing agents. Since the 
snugly fitting earplug is maintained in intimate 
contact with the lining of the ear canal, the 
plug must be made from a nontoxic material. 
The material must be compliant not only for 
comfort but also in order that a good seal may 
be established. The material should not be per¬ 
manently deformed as the result of wear. From 
the practical point of view of economical pro¬ 
duction and distribution, the material must be 
easily molded. The percentage of earplugs de¬ 
fective because of the failure of the material 
to mold, or from stripping the molds of the 
earplugs, should be small. 

The most satisfactory results thus far ob¬ 
tained at the Psycho-Acoustic Laboratory were 
achieved with polymerized chloroprenes, e.g., 
neoprene, which, when formulated with a maxi¬ 


mum of nontoxic plasticizer, yield ear wardens 
that are soft, reasonably resilient, and mark¬ 
edly resistant to the action of earwax. Prom¬ 
ising also are the polyvinyl-chloride-copolymers, 
e.g., Vinylite. The chief technical problem with 
this material centers about the choice of plasti¬ 
cizer, since some plasticizers are irritant, some 
are ineffective at low temperatures, and some 
are generally extracted from the warden by 
contact with earwax. 


263 Acoustic Insulation 

The amount of acoustic insulation afforded 
by an earplug can be evaluated either by the 
threshold technique or by the method of loud¬ 
ness balances. By the threshold technique the 
observers make measurements monaurally, 
using the right ear. The left ear is carefully 
blocked off, and the earplug to be tested is 
inserted by the observer into his right ear. He 
is then seated in a chair in an anechoic room 
with his right ear toward a loudspeaker one 
meter away. The observer adjusts the intensity 
of the test tones until he determines his thresh¬ 
old of hearing. 

In order to evaluate the amount of insulation 
for any frequency, it is necessary to determine 
the amount by which the open-ear threshold is 
shifted when the earplug is placed in the ear. 
Two series of measurements are therefore nec¬ 
essary. One threshold measurement is made at 
each frequency for the open ear and another at 
each frequency after the ear has been plugged. 
The difference between the two intensities is 
the amount of insulation afforded by the ear¬ 
plug. 

In Figure 54 is presented a comparison of 
tan neoprene, black neoprene, and Vinylite 
models of the V-51R ear warden. For all prac¬ 
tical purposes the three curves are identical. 
Acoustic insulation values range from approx¬ 
imately 26 db at the low frequencies to 40 db 
at the high frequencies. 

In addition to the V-51R ear warden, the 
following earplugs were also examined at the 
Psycho-Acoustic Laboratory (see Figure 55). 

A. Baum’s Mega eardrum protectors. Nipple¬ 
shaped, soft rubber capsules, bearing four 



AURAL PROTECTIVE DEVICES: EAR WARDENS 


43 


longitudinal grooves. Each pair of protectors is 
connected by a long, stout thread. One size. 

B. Nods noise mufflers. A cylinder of latex 
sponge impregnated through about two-thirds 



Figure 54. Comparison of acoustic insulation 
for three models of the V-51R ear warden. Data 
obtained by threshold technique. 


The flare at the base limits the depth of inser¬ 
tion. Five sizes. Very similar devices have been 
found on captured German soldiers and in 
Japanese military stores. The intent of the de¬ 
sign is an acoustic valve. 

I. MSA ear defenders. A capsule of gum 
rubber encloses a metal insert of the shape 
illustrated. Three sizes. 



of its length with antiseptic wax. Kneaded with 
the fingers to proper size. 

C. Olygo noise absorbers. A cylinder of cot¬ 
ton impregnated with antiseptic wax, kneaded 
with the fingers to proper size. 

D. Flents antinoise ear stopples. A cylinder 
of cotton impregnated with antiseptic wax, 
kneaded with the fingers to proper size. 

E. Nelson’s ear stoppers. A hard spherical 
globule of colorless plastic terminated in a 
T-shaped handle which facilitates insertion and 
removal. Three sizes. 

F. Sepco safety ear protectors. A bullet¬ 
shaped capsule made of rubber, or rubber-like 
synthetic material, is filled with flocculent rub¬ 
ber enclosed with a plug of rubber sponge. An 
external flange on the cylindrical body of the 
plug limits the depth of insertion in the ear 
canal. Near the rounded tip of the plug there 
is a minute aperture or puncture. Five or more 
sizes. Used with antiseptic lubricant jelly. 

G. SMR ear stoppers. These are similar to 
the Sepco safety ear protectors. Five or more 
sizes. Used with antiseptic lubricant jelly. 

H. Mallock-Armstrong ear defenders. These 
are hard, black, plastic plugs. They have a 
channel through them which is terminated at 
the outer end by a cellophane-like diaphragm 
placed between two layers of fine wire screen. 



J. NDRC ear wardens (Type V-29R). These 
are capsules of soft neoprene. Like the MSA 
ear defenders, they have two concentric flanges. 
A safety valve is provided to limit the depth of 
insertion. A tab at the base facilitates removal. 
Three sizes. 















































44 


SOUND CONTROL 


The resulting insulation data for these ten 
earplugs are given in Figure 56. For purposes 
of comparison, the data include the results ob¬ 
tained by the common expedient of stuffing the 
ear canal with cotton. All the earplugs tested, 



FREQUENCY IN CYCLES PER SECOND 



FREQUENCY IN CYCLES PER SECOND 


Figure 56. Acoustic insulation provided by the 
earplugs pictured in Figure 55. Cotton is in¬ 
cluded for purposes of comparison. 


with the possible exception of Baum’s Mega 
eardrum protectors, have good acoustic insula¬ 
tion for the range of frequencies from 2,000 to 
10,000 c. Over the low-frequency range, where 
protection is hard to attain, the various ear¬ 
plugs differ considerably in the amount of 
acoustic insulation afforded. 

The validity of the threshold method of deter¬ 
mining acoustic insulation has been challenged 
from time to time. One way of testing the in¬ 
sulation at higher levels of noise is by the 
method in which a loudness balance is obtained 
between sounds introduced into the two ears 
by two separate earphones. The observer con¬ 


trols the level of the test signal in one earphone 
until the sounds in the two ears appear equally 
loud. After the balance is obtained, an earplug 
is inserted into one of the ears, and the loudness 
is again balanced. The amount, in decibels, by 
which the intensity of the sound must be in¬ 
creased in order to appear equally loud is taken 
as a measure of the insulation afforded. 

The results of the loudness-balance measure¬ 
ments of the insulation of the V-51R ear war¬ 
dens are presented in Figure 57. In addition 
to the curve for loudness balance, a curve of 
insulation measurements for the same observers 



Figure 57. Comparison of acoustic-insulation 
curves obtained by the loudness-balance and 
threshold methods. 


using the threshold technique is shown for 
comparison. The latter curve shows that the 
magnitude of acoustic insulation is essentially 
the same at high intensities as at low intensities 
of noise. 


Articulation Measurements 

The results of articulation tests show that 
with most Service interphones, operating at 
normal amplifier gains and with normal voice 
levels, the wearing of ear wardens does not 
impair the reception of speech. On the contrary, 
reception of speech may be improved. Figure 
58 shows the results obtained for articulation 
tests performed in the quiet and with various 
amounts of ambient noise introduced into the 
test room. Speech of weak intensity, in the quiet 
and in noise levels below 75 db, is not heard as 





















































































































































45 


AURAL PROTECTIVE DEVICES: EAR WARDENS 


readily by listeners wearing ear wardens. In 
the quiet, however, and for all noise levels, 
speech slightly above conversational level (75 
to 80 db) is heard equally well by listeners with 
and without ear wardens. In noise levels above 
75 db the intelligibility of speech, as is shown 
by Figure 58, is better with ear wardens. A 


and cause permanent degeneration of the deli¬ 
cate sensory structures in the inner ear. 

The problem of protection against gun blasts 
differs slightly from that of protection against 
continuous unwanted noise. A blast may be 
described in physical terms as a single sound¬ 
wave, a “transient,” of unsymmetrical form. 



Figure 58. The relation between articulation and sp eech level with noise level as the parameter. In loud 
noise ear wardens improve articulation. 


signal that is audible to the open ear in a noise 
higher than 75 db is equally audible or more 
audible when ear wardens are worn. In the 
noise levels usually encountered in submarine 
engine rooms, tanks, and airplanes, ear wardens 
should improve speech communication. 

2 6 3 Blast Exclusion 

The disastrous effects on unprotected ears of 
the discharge of nearby artillery or the close 
burst of a large caliber shell are well known. 
Severe exposure in front of a large gun or really 
close escape from a shell burst or land mine 
ruptures the eardrums and may also disrupt 


The major wave is a single violent but brief 
wave of positive pressure amounting to many 
pounds to the square inch and greatly exceeding 
any pressures reached by sustained sounds. 
The positive pressure wave is followed by a 
longer but less intense wave of negative pres¬ 
sure, and by smaller secondary waves. Fortu¬ 
nately, the ear can fairly well tolerate intense 
single waves which would rapidly cause severe 
damage if they were repeated several hundred 
times a second, as in a steady tone. But there 
is a limit beyond which a blast becomes strong 
enough to produce temporary deafness, particu¬ 
larly if it is a sharp blast with a steep initial 
wave front. 









46 


SOUND CONTROL 


In order to protect the ear against blast 
waves the pressure wave must be attenuated, 
and the problem is similar to the problem of 
attenuation of any other noise in the audible 
range. The same considerations of comfort, 
wearability, simplicity, and cleanliness that 
apply to defenders against continuous noise 
apply equally to protectors against blasts. It is 
desirable, however, that the earplugs should not 
be moved bodily inward by the pressure wave. 

The NDRC ear wardens, Type V-51R, were 
originally designed for protection against con¬ 
tinuous noise rather than against blasts. They 
nevertheless fulfill most, if not all, of the obvi¬ 
ous requirements of blast protectors. The de¬ 
sign of the ear warden, with external safety 
tab and a large flange in close contact with the 
wall of the ear canal, effectively prevents its 
being driven into the ear canal by the pressure 
wave of a blast. 

Testing the ear warden in the presence of 
gunfire in the field is not only the most practical 
and economical but also the most conclusive 
kind of test of its ability to protect the ear from 
blasts. Accordingly, a number of field tests 


were arranged. Through the courtesy of the 
Field Artillery Board, members of the Psycho- 
Acoustic Laboratory were privileged to attend 
an informal test of the V-51R ear warden in 
the immediate vicinity of some of the heavier 
armament at Fort Bragg, North Carolina. 
After the firing, a short questionnaire was 
given to all men participating in the field test. 
On the basis of these results, the Field Artillery 
Board concluded that the V-51R ear wardens 
“are suitable as replacements for cotton or 
cotton waste by members of the firing battery.” 
After similar tests, the Chemical Warfare 
Board also concluded that ear wardens are 
superior to cotton in protecting the wearer 
against blast effects, and that communication 
is not impaired when ear wardens are worn. 

Following formal or informal trials of the 
wardens, and examination of a considerable 
number of commercial earplugs, procurement 
of neoprene or Vinylite wardens was under¬ 
taken in behalf of the United States Navy, the 
Royal Navy, the Royal Canadian Navy, the 
United States Army Air Forces, and the Chem¬ 
ical Warfare Service, U. S. Army. 



Chapter 3 

SOME CHARACTERISTICS OF THE HUMAN EAR 


O NCE it had been determined (see Section 
2.5) that the only major effect of loud 
noises on man was interference with normal 
auditory perception, the research endeavors of 
the Electro-Acoustic and Psycho-Acoustic 
Laboratories were directed toward the more 
pressing problems of communication. Since 
speech-communication systems usually termi¬ 
nate in human ears, it will facilitate discussion 
in the following chapters if some of the funda- 


an area at the opening of approximately 0.4 
sq cm. Pinna and external canal together are 
referred to as the outer ear. 

The canal is closed at its inner end by the 
cone-shaped eardrum (tympanic membrane). 
Internal to this membrane is the middle ear, 
enclosing a cavity of 1 to 2 cu cm volume and 
containing the three small bones (ossicles) 
which transfer the agitations of the eardrum 
to the inner ear. Pressure on the two sides of 



AURICLE 


SEMICIRCULAR CANALS 


AUDITORY 

NERVE 


EXTERNAL 

AUDITORY 

CANAL 


EUSTACHIAN 

TUBE 


Figure 1. Schematic drawing of the ear. 


mental facts about the ear are here made ex¬ 
plicit. (Several comprehensive treatments of 
hearing are available.) 3 - 5 - 8 

A schematic drawing of the ear is shown in 
Figure 1. The visible portion of the ear (auricle 
or pinna) is a fibrocartilaginous structure lead¬ 
ing to the external auditory canal. This canal is 
from 2.0 to 2.5 cm long, approximately 0.7 cm in 
diameter, with a volume of about 1.0 cu cm and 


the membrane is equalized by the Eustachian 
tube, which opens during the acts of swallowing 
or yawning. 

The inner ear (cochlea and semicircular 
canals) is filled with liquid and surrounded by 
bone. Acoustic stimulation is transmitted via 
the ossicular chain to the liquid of the cochlea 
which, in turn, causes movements of a mem¬ 
brane (basilar membrane) supported in the 


47 





48 


SOME CHARACTERISTICS OF THE HUMAN EAR 


liquid. The auditory nerve endings are distrib¬ 
uted along this basilar membrane. Presumably, 
different nerve endings respond to different fre¬ 
quencies, depending upon their location along 
the membrane, and in this way the inner ear 
can operate as an acoustic analyzer. 

Thus the acoustic stimulus is transmitted 
through the auditory canal to the drum and is 
converted into mechanical movement of the 
bones of the middle ear. This movement leads 
to hydrodynamic action in the inner ear which 
results, finally, in the electrochemical event 
known as a nerve impulse. Nerve impulses 
travel along the eighth cranial nerve to the cen¬ 
tral nervous system and there, by ways not 
understood at present, lead to sensations of 
hearing. The actual course of events can be fol¬ 
lowed with some confidence as far as the 
tympanic membrane; beyond that, the details 
become controversial. 


31 SOUND-PRESSURE DISTRIBUTION 
IN THE AUDITORY CANAL 

The physical stimulus acting on the ear can 
best be specified by measuring the sound pres¬ 
sure at the eardrum. Resonance effects in the ear 
canal and diffraction effects around the head 
are thereby taken into account. An experi¬ 
mental study 17 was made with the object of 
determining these effects and also the varia¬ 
tions, as a function of frequency, of the sound 
pressure along the auditory canals of a number 
of subjects (male and female) placed in a plane 
progressive sound field. 

A small, flexible, probe microphone was in¬ 
serted at various positions along the length of 
the auditory canal, and the sound pressure de¬ 
termined as a function of position and fre¬ 
quency. The subjects were placed in the sound 
field of a loudspeaker in a room free from acous¬ 
tic wall reflections (anechoic chamber). The 
free-field sound pressure at the subject’s loca¬ 
tion was also determined. The ratio of the sound 
pressure at the eardrum to the free-field sound 
pressure was taken to be a measure of the com¬ 
bined obstacle effect of the head and resonance 
effects in the ear canal. In addition, sound-pres¬ 
sure measurements were made at the entrance 


of the ear canal and at a point about halfway 
down its length. These measurements serve, to 
some extent, to separate the resonance and dif¬ 
fraction phenomena and to explore the pressure 
distribution in the auditory canal itself. The 
results are plotted in the form of pressure ratios 
as a function of frequency for values of azimuth, 
4> (the angle between the loudspeaker axis and 
the plane of symmetry through the subject’s 
head), ranging from 0 to 90 degrees. 

The subject was seated in a chair 6 ft away 
from the loudspeaker, and his head was secured 
by a head clamp mounted on the back of the 
chair. The clamp was designed (see Figure 2) 
to permit the fixing of the subject’s head with 
a minimum of discomfort. This is necessary in 
order to make possible the insertion of the 



Figure 2. Sound-pressure measurement in the 
auditory canal. 

probe microphone into the auditory canal with 
a minimum of danger due to accidental shifts 
in head position. The microphone used in these 
tests consisted of a Western Electric Type 
640-AA condenser microphone to which a probe 
tube of small diameter was coupled by means 
of suitable coupling rings (see Figure 2). About 






SOUND PRESSURE IN EAR CANAL 


49 


half the total length of the tube consisted of 
brass tubing; the rest was made up of flexible 
plastic tubing. It is this feature, incorporating 
sufficient softness and flexibility, which is es¬ 
sential for the application at hand. 

To permit easy and accurate insertion of the 
microphone into the ear canal, the microphone 
holder was mounted in a special adjustable 
carriage (see Figure 2). The carriage is de¬ 
signed to permit easy and free adjustments in 
three orthogonal directions. The chief problem 
involved in the measurements was the insertion 
of the probe tube into the ear canal to the point 
where its open end would be “at the eardrum.” 
This was achieved by thrusting the probe gently 
into the canal by means of the lateral motion 
adjustment of the holder. The subject was in¬ 
structed to advise the experimenter as soon as 
(1) any pain or (2) any auditory sensation was 
experienced. Experience has shown that when 
the eardrum is contacted, an auditory sensation 
is experienced which has the character of a dull 
thump. After the subject reported contact with 
the eardrum, the probe was withdrawn about 
1 mm and measurements were begun. 



Figure 3. Ratio of sound pressure at the ear¬ 
drum to sound pressure in the free field at the 
center of observers’ head. Average of 6 to 12 male 
left ears. 


Figure 3 shows, for six to twelve male sub¬ 
jects, the average ratio of the sound pressure 
at the eardrum to the free-field sound pressure, 
plotted as a function of frequency. It is evident 
from the figure that the human ear is an effec¬ 
tive acoustic “amplifier,” with maximum ampli¬ 
fication of the order of 20 db near 3,000 c. The 
effect is somewhat dependent on azimuth, and is 


markedly a function of frequency. Figure 4 gives 
an estimate of the variation with azimuth in the 
form of a polar diagram. For the range of azi¬ 
muths investigated, the maximum pressure 



Figure 4. Variation of sound pressure at the 
eardrum with azimuth </>, for constant free-field 
sound pressure. Average of 6 to 12 male ears. 


ratio is obtained near <f> = 90 degrees, where the 
subject has his ear turned toward the sound 
source. 

Figure 5 shows the pressure distribution in 
the ear canal itself, expressed in terms of the 



Figure 5. Ratio of sound pressure at the ear¬ 
drum to sound pressure at the entrance of the 
auditory canal. Average of 6 to 12 male left ears. 


ratio of the sound pressure at the eardrum to 
the pressure at the entrance of the canal. A 
resonant peak of about 10 db is reached at about 
4,000 c, quite independent of azimuth. It can be 
shown 17 that diffraction around the head and 
pinna accounts for only the smaller part of the 








































































































50 


SOME CHARACTERISTICS OF THE HUMAN EAR 


total acoustic amplification. Resonance in the 
auditory canal contributes most of the pressure 
increase at the drum. A few measurements on 
female subjects indicated no essential difference 
in the acoustic behavior of the ear canal as com¬ 
pared with the measurements made on male 
subjects. 


3 2 THE SENSITIVITY OF THE EAR 

The absolute sensitivity of the ear is defined 
as the minimum sound pressure necessary to 
evoke an auditory experience. The point at 
which the sound pressure is measured must be 
specified in every case. As was pointed out in 
the preceding section, the sound pressure at the 
eardrum usually exceeds the free-field sound 
pressure producing it. Hence the thresholds will 
be different if expressed in terms of minimum 
audible pressure at the eardrum [MAP] or 
minimum audible free-field pressure [MAF] . 6 In 
particular, the MAF is lower than the MAP 
produced by an earphone worn by the observer. 

The differences between MAP and MAF 
found' 5 at medium and high frequencies can 
probably be explained on the basis of the probe- 
tube measurements described in the preceding 
section. No entirely satisfactory explanation, 
however, has yet been offered of the difference 
of about 10 db found 6 at frequencies as low as 
100 c. 

The sound pressure at the threshold of audi¬ 
bility varies considerably for different frequen¬ 
cies (see Figure 6). At its best the ear is sensi¬ 
tive to frequencies between 20 and 20,000 c, but 
the region of maxium sensitivity is for fre¬ 
quencies between 1,000 and 4,000 c. At these 
frequencies the MAP approaches the reference 
level of 0.0002 dyne/cm- adopted by the Ameri¬ 
can Standards Association. 11 


3 - 2 .i Difference between Monaural and 
Binaural Sensitivity 

It has been argued 7 that the binaural thresh¬ 
old is equal to the threshold of the more sensi¬ 
tive of the two ears. When the two ears differ 
substantially in sensitivity this argument ap¬ 


plies, but experiments at the Psycho-Acoustic 
Laboratory indicate that, when both ears have 
approximately the same sensitivity, the bin¬ 
aural threshold for pure tones is about 3 db 
lower than the monaural. 

A lower binaural threshold was obtained 
under two experimental conditions. In the first 
experiment, the thresholds for single ears were 
compared with the binaural threshold obtained 
with the same input to both earphones. In order 
to eliminate any effects of small differences in 



Figure 6. Monaural threshold for the average 
normal ear. 


the sensitivity of the two ears, a second experi¬ 
ment established the two monaural thresholds 
and then determined the binaural threshold 
with the input to the two earphones adjusted 
separately to each of the monaural threshold 
values. In this way the sensation level (see 
Section 3.3.2) for both ears was made the same. 
This latter method is superior for estimating 
the difference between monaural and binaural 
sensitivity, although both methods indicated 
that the binaural threshold for pure tones in 
quiet is lower than the monaural threshold. 


Altitude Effects on Sensitivity 

The monaural threshold of hearing was de¬ 
termined at sea level and at 35,000 ft for fifteen 
subjects listening with earphones. 13 When cor¬ 
rections were made for the effects of the re¬ 
duced ambient pressure on the earphone re¬ 
sponse and on the cushion leaks, no change in 
the auditory threshold was found. 


































PITCH AND LOUDNESS 


51 


3 3 PITCH AND LOUDNESS 

It is commonly assumed that a direct, linear 
relation exists between apparent pitch and 
frequency and between apparent loudness 
and physical intensity. Experimental evidence 
shows that such assumptions are unjustified. 


Relation of Pitch to Frequency 

When listeners are instructed to adjust the 
frequencies of pure tones until they seem to be 
separated by equal pitch distances, the result¬ 
ing scale of apparent pitch does not coincide 
with the scale of musical intervals. The mel 
has been chosen as the unit of apparent pitch 
and is so defined that a frequency of 1,000 c 
produces a tone of 1,000 mels. 10 A tone of 2,000 
mels, for example, is twice as high in pitch as 
one of 1,000 mels and is produced by a fre¬ 
quency of 3,120 c. The pitch scale is shown in 
Figure 7, where pitch in mels is plotted as a 
function of frequency. 



Figure 7. Pitch in mels as a function of fre¬ 
quency. Equal mel intervals represent subjec¬ 
tively equal pitch distances. 


There is reasonably good agreement between 
this scale of pitch and the function which re¬ 
lates the position of maximum agitation on the 
basilar membrane to the frequency of the 
acoustic stimulus. The hypothesis which fol¬ 
lows naturally from this agreement is that 
equal pitch distances represent equal distances 
along the basilar membrane. The scale can also 
be used to predict the sensitivity of the ear to 


small changes of frequency (differential sensi¬ 
tivity to frequency). A third correspondence 
appears between the pitch function and certain 
calculations used at the Bell Telephone Labora¬ 
tories 12 for predicting the results of articulation 
tests. According to these calculations, equal 
pitch distances make approximately equal con¬ 
tributions to intelligibility. 

The various suggested hypotheses have not 
been fully and rigorously tested, however, and 
the final unification of these several aspects of 
the auditory function awaits further research. 


3 3 2 Relation of Loudness to Intensity 

The physical intensity of a sound is usually 
expressed in decibels referred to some arbitrary 
value. The reference value determines the na¬ 
ture of the scale, and three different kinds of 
reference levels are commonly used. The inten¬ 
sity level of a sound is the number of decibels 
that the intensity of a free, progressive sound 
wave is above the arbitrary reference intensity. 
On this scale, 0 db = 10 -16 watt/cm 2 = 0.0002 
dyne/cm 2 . The sensation level indicates the 
number of decibels which the sound must be 
attenuated to reach the listener’s threshold of 
hearing. The reference value in this case is the 
threshold, and conversions can be made from 
sensation level to intensity level when the 
threshold value is known. 

The loudness level is the intensity level of a 
1,000-c tone which is judged to be of the same 
apparent loudness as the given sound. The phon 
is the unit of loudness level. A loudness contour, 
for example, is given by all the tones which are 
equal in loudness and have the same value in 
phons. The magnitude of the psychological dif¬ 
ference between one loudness level and another 
is not specified, however. Experiments have 
determined this scale of loudness, and the sone 
has been suggested as the psychological loud¬ 
ness unit. 8a The sone is so defined that a tone 
of 1,000 c at a loudness level of 40 db produces 
a psychological experience having the magni¬ 
tude of one sone. On this scale, for example, a 
tone at a loudness level of 70 phons is reduced 
to one-half its apparent loudness if the inten¬ 
sity is reduced by approximately 10 phons. 





































52 


SOME CHARACTERISTICS OF THE HUMAN EAR 


34 MASKING 

For purposes of military communication in 
the presence of noise, one of the most important 
characteristics of the ear is its ability, as an 
analyzer, to respond selectively to certain com¬ 
ponents of the total acoustic pattern and to 
ignore others. This selective mechanism can 
be studied in its simplest form when two pure 
tones are introduced into the ear and the lis¬ 
tener is asked to report the presence or absence 
of one or the other of the tones 1 (see Section 
8.1.1). In such experiments it is found that the 
ear is not a perfect analyzer, but that some 
tones obscure the perception of others. The 
extent to which the signals interfere is called 
the masking of the one tone by the other. 

The term masking is customarily defined as 
the shift in the threshold of hearing for pure 
tones which is produced by the introduction 
of an extraneous interference. The amount of 
masking is then measured in terms of the in¬ 
crease in the intensity of a tone that is required 
to make it heard in the presence of the inter¬ 
ference. This notion can be readily extended 
to include speech as well as tones. The intro¬ 
duction of an interfering noise makes it neces¬ 
sary to raise the speech intensity in order for 
it to be understood. The necessary increment 
in the speech intensity is taken as the measure 
of the masking produced by the noise. If the 
masked threshold is defined as the sound pres¬ 
sure necessary to make a pure tone just audible 
in the presence of an interfering sound, then 
the amount of masking at that frequency is 
taken as the difference in decibels between the 
quiet and the masked thresholds. It is some¬ 
times convenient to plot the absolute threshold 
in quiet as a straight line and to indicate the 
masking at each frequency in decibels relative 
to this line. Such a plot of masking against 
frequency is referred to as the masking audio- 
gram for the interference used. 


Masking of Tones by Noise 10 

In order to determine the masking effects of 
random noise on pure tones, four experienced 
listeners with normal hearing were tested indi¬ 


vidually in a quiet, electrically shielded room. 
Tones and noise were presented monaurally, and 
a frequency calibration for each listener was 
made with a small probe tube placed about 
2 mm from the face of the earphone. The sub¬ 
jects determined their thresholds by adjusting 
an attenuator which controlled the intensity of 
the tone. Thresholds were obtained for 16 fre¬ 
quencies between 100 and 9,000 c for quiet 
conditions and for eight levels of masking 
noise. The eight noise levels varied from a sen¬ 
sation level of 20 to 90 db. Frequency analysis 
of the noise spectrum showed that the corre¬ 
sponding spectrum levels (level per cycle) of 
the noise varied from approximately —10 to 
60 db. 

When the masked thresholds are corrected 
for the frequency response of the earphone, the 
idealized masking contours shown in Figure 8 



FREQUENCY IN CYCLES PER SECOND 

Figure 8. Masked-threshold contours. These 
curves show the monaural thresholds for pure 
tones when masked by various levels of white 
noise having uniform energy per cycle. 

are obtained. The contours tend to be parallel, 
and are separated by intervals of approximately 
10 db, corresponding to the intervals between 
the masking noise levels. The functions for the 
four highest noise levels are very similar. The 
ratio between the masked threshold of a given 
tone and the level per cycle of the noise at that 
frequency is the same for these top four con¬ 
tours, and in Figure 9 the average difference 
between the signal level and the level per cycle 
of the masking noise is plotted against fre¬ 
quency. The points represent the values ob¬ 
tained from the present experiment; the smooth 
curve is from data obtained at the Bell Tele- 






























































MASKING 


53 


phone Laboratories. 12 Thus we see that in order 
to be just audible, a tone of 1,000 c must be 
18 db above the level per cycle of the noise. 

The function in Figure 9 is sometimes taken 
as defining the “critical bandwidth” of a mask¬ 
ing noise. Presumably it reveals what band of 
frequencies in a flat masking noise is effective 


Sis 
s - * 

s | o 

4 P 


65 J 





























* 


















. 






• 






































































3 4 5 6 7 6 9 8 3 456769 

1000 10000 
FREQUENCY IN CYCLES PER SECONO 


Figure 9. Ratio between the monaural masked 
threshold of a pure tone and the level per cycle 
of the masking noise measured at the frequency 
of the pure tone. 


in masking a pure tone located at the center 
of the band. This interpretation rests on the 
assumption that the total energy required to 
mask the pure tone is equal to the energy of 
the tone itself. Since the energy in a band of 
white noise is proportional to the width of the 
band, it can be determined from Figure 9 how 
wide the band of noise would need to be in 
order for its total energy to be equal to that 
of a given pure tone at its masked threshold. 
For example, at 1,000 c the level per cycle of 
the masking noise is 18 db below the masked- 
threshold level of the pure tone, and since 18 db 
corresponds to an energy ratio of 63 to 1, the 
band of frequencies having a total energy equal 
to that of the 1,000-c tone would be 63 c wide. 

The curve of Figure 9 is of further theoretical 
interest 9 because of the rather close agreement 
between the widths of the critical bands (ex¬ 
pressed in cycles per second) and the widths 
of equal pitch intervals 50 mels wide (see Sec¬ 
tion 3.3.1). 

These data can be considered from another 
point of view. It is evident from Figure 8 that, 
except near the quiet threshold, the masked 
threshold goes up 10 db when the level of the 


noise is raised 10 db. Figure 10 shows the 
relation between masking ( M ) in decibels and 
the effective level ( Z ) of the masking noise. 
The effective level Z is the difference in decibels 
between the pure-tone threshold in quiet and 
the total energy level in one critical band. In 
other words, Z is equal to the sensation level 
of a pure tone having the same energy as the 
critical band of frequencies which is just able 



Figure 10. Relation between masking ( M ) and 
the effective level ( Z ) of the masking noise. 


to mask the tone. Observations at the Psycho- 
Acoustic Laboratory indicate that Z may be 
interpreted simply as the sensation level of the 
critical band. The datum points represent the 
empirical values for the frequencies, 350, 500, 
1,000, 2,800, 4,000, and 5,600 c. The curve 
drawn through these points has a slope of one 
for all values of Z greater than 10 db. 


3-4-2 Effect of Duration on the Masked 
Threshold of Tones 

The investigations discussed in the preceding 
section considered only tones of relatively long 
duration. The effect of shortening the duration 
of the tone has also been studied. 16 ® The results 
indicate that, as the duration of the tone is in¬ 
creased up to one-fifth of a second, the tone 
becomes more audible by a proportional amount. 
The energy required for a short tone to be 
heard is inversely related to its duration. For 




































54 


SOME CHARACTERISTICS OF THE HUMAN EAR 


durations longer than one-fifth of a second, the 
increase in audibility is relatively less as the 
duration is lengthened. Tones of one-half sec¬ 
ond duration or longer may be regarded for 
practical purposes as equivalent to tones of 
infinite duration. The signal-to-noise ratio 
(where the noise is expressed in terms of the 
level per cycle) as a function of the tonal 
duration is plotted in Figure 11 for four pure 


Masking of Complex Tones: 

Applications to Radar 

For most military applications, the concern 
is with the recognition of complex rather than 
pure tones. The auditory reception of radar 
echoes, for example, presents the ear with a 
train of pulses which differ considerably in 
quality from pure tones. A pulse of short dura- 


CD 

O 



SIGNAL DURATION (MILLISECONDS) 

Figure 11. The relation between the signal-to-noise ratio ( S/N ) and the duration of a pure tone at the 
level of the masked threshold. N is expressed as the noise energy per cycle. Each plotted point is the 
average S/N for four levels of masking noise (50, 70, 90, and 110 db SPL). Each symbol is for a different 
frequency as indicated. 


tones of different frequencies. Since the shape 
of the function is independent of the level of 
the masking noise, the points of Figure 11 were 
derived by taking the average for the four 
highest noise levels used. Data obtained in the 
quiet were less reliable, but seemed to show the 
same relation. The curve as drawn represents 
the average for the four frequencies and thus 
may be considered as a generalized relation be¬ 
tween duration and threshold for all frequen¬ 
cies and noise levels. The use of Figure 11 in 
conjunction with Figure 9 permits an approx¬ 
imate prediction of the threshold for tones of 
any frequency and duration when masked by a 
relatively intense random noise of known spec¬ 
trum. 


tion is found to contain many harmonics of the 
fundamental frequency, and if the pulse dura¬ 
tion is short enough (50 qsec or less), all the 
effective audible harmonics are present in equal 
amounts. 

During the war the enemy often detected 
our radars by auditory means. It was necessary, 
therefore, to determine what factors contribute 
to the recognizability of radar pulses and to 
examine the possibility of disguising the radar 
in such a way as to make auditory detection 
more difficult. 

The threshold of audibility for pulses of 
10 [.isec duration was determined for pulse 
repetition frequencies [PRF] from 1 to 8,000 
pulses per second [pps]. A dynamic receiver 













































DIFFERENTIAL SENSITIVITY 


55 


(PDR-10) was used, which gave a fairly uni¬ 
form response up to 8,000 c. For PRF’s above 
approximately 1,000 pps the shape of the 
threshold function is similar to that obtained 
with sine waves. From 1,000 down to about 
10 pps, however, the root-mean-square [rms] 
.sound-pressure level at threshold remains ap¬ 
proximately constant. For a PRF slower than 
about 6 pps, the individual pulses are heard as 
isolated clicks, and the threshold depends only 
upon the peak amplitude of a single pulse. But 
for the frequencies where discrete pulses are 
not heard, the ear apparently responds to the 
higher harmonics which fall in the frequency 
region to which it is more sensitive. Thus the 
shape of the threshold function for a complex 
tone may differ considerably at low frequencies 
from that for pure tones. 

The recognition of a radar signal depends 
primarily upon three psychological factors: the 
audibility, the tonality, and the regularity of 
the incoming signal. 14 Regularity depends upon 
the sweep rate of the radar and may be modified 
by varying the sweep. Audibility and tonality 
are more complex. The operator listening for 
radar signals must contend with noise, and the 
ease of detection is dependent to a great extent 
on the noise level at the time. 

The presence of noise suggests that there 
might be some advantage in randomizing the 
pulse rate. Methods for randomly triggering a 
radar set have been investigated in the Psycho- 
Acoustic Laboratory. A random PRF loses ton¬ 
ality, blends into the background noise, and is 
more easily masked. Laboratory tests showed 
that random modulation of the interval between 
pulses could reduce the audibility as much as 
4 db. The tonality of the signal then changes 
markedly from the buzz of pulses to complete 
noisiness. When the PRF is modulated by bands 
of noise containing frequencies from 20 c to 
one-half the PRF, it is found that all ranges 
of modulation beyond 50 per cent are judged 
the same in tonality and that greater ranges 
of modulation than 50 per cent do not increase 
the noisiness. 

Although the laboratory tests were promising 
and the results of considerable interest for pre¬ 
dicting the audibility of complex tones in noise, 
field tests conducted at the U. S. Submarine 


Base, New London, Connecticut, indicated that 
even the modulated signal is immediately rec¬ 
ognized as a radar whenever the signal is 
strong enough to saturate the receiver. Since, 
for the receiver tested, there was only a 6-db 
range between saturation and inaudibility of 
the steady signal, there was too small a range 
over which the random pulsing could have an 
effect. 

3 5 DIFFERENTIAL SENSITIVITY 

Differential sensitivity refers to the change 
in the stimulus which is necessary in order to 
be judged by an observer as just noticeably 
different. Thus, for example, the smallest dif¬ 
ference in frequency which can be detected by 
ear defines the differential threshold for fre¬ 
quency. 

Differential sensitivity to frequency is a 
function of both frequency and intensity. 4 
Above 500 c the relative differential thresholds 
(A F/F) are approximately constant, but below 
500 c the absolute thresholds (AF) are nearly 
constant. For all frequencies, the maximum 
sensitivity is obtained at sensation levels of 
30 db or more. The value A F/F is smallest for 
tones of about 2,000 c at sensation levels of 30 
to 80 db. 

The differential threshold for intensity like¬ 
wise depends upon the frequency and intensity 
of the tones. 2 At sensation levels below 20 db, 
sensitivity varies considerably as a function of 
frequency, with maximum sensitivity in the 
region of 2,500 c. The number of decibels re¬ 
quired to perceive an increment in loudness 
decreases for the higher sensation levels, and 
differences as small as one-fifth of a decibel are 
detectable under optimal conditions. 

The relative differential sensitivity of the ear 
is greatest for both frequency and intensity 
over the frequency range which corresponds to 
the region of maximum absolute sensitivity. 

3-51 Differential Sensitivity to Noise 15 

Under some circumstances it is possible to 
harness the noises of mechanized warfare to 
beneficial effect. In certain tactical maneuvers 
there is considerable advantage to be gained 



56 


SOME CHARACTERISTICS OF THE HUMAN EAR 


through knowledge of the sounds made by ve¬ 
hicles in motion. Cases in point might be: esti¬ 
mation of changes in engine noise as indicating 
size, direction, speed of approach, or retreat of 
plane formations; interpretation of noise vari¬ 
ations picked up by submarine detection ap¬ 
paratus; relation of changes in air turbulence 
and propeller noises to changes in speed and 
altitude of an airplane. Such instances encour¬ 
aged the development of two phonographically 
recorded listening tests: a pitch test (Auditory 
Test No. 6) which measures the listener’s abil¬ 
ity to detect frequency variations in a noise 
spectrum, and a loudness test (Auditory Test 
No. 7) for intensity variations. 

The pitch of a band of frequencies is rather 
indefinitely localized, but appears to be some¬ 
where in the middle range of the frequencies 
present. To construct a test of differential sen¬ 
sitivity to the frequencies of a noise, it is neces¬ 
sary therefore to make the apparent center 
move up and down by small amounts. Such 
changes must be made without an accompany¬ 
ing change in intensity. To accomplish this! the 
circuit shown schematically in Figure 12 was 
used. 



Figure 12. Schematic circuit used to record the 
pitch test. 


Each item of the pitch test calls for a com¬ 
parison of the pitch of two noises. For each 
item, therefore, the first of the two noises is 
the “standard,” and remains the same through¬ 
out the test. The second noise, the “variable,” 
may be higher or lower in pitch than the stand¬ 
ard. The center frequency of the standard noise 
was approximately 1,300 c and had the spec¬ 


trum shown in Figure 13. Since the center 
frequency of the bands is difficult to assign 


































- 




> 

\ ^ 





/ 

Y 


y' 







\ 

N 




-V - 

f / 











































‘coo ' * *0,000 

FREQUENCY IN CYCLES PER SECOND 


Figure 13. Showing the shape of the noise 
spectra employed for the test of pitch discrimina¬ 
tion. The result of shifting the spectrum up or 
down the frequency scale is shown by the curves 
marked “Lower” and “Higher.” 

precisely, it is convenient to pick some arbi¬ 
trary frequency at a distance from the center 
frequency and to measure the frequency change 
in terms of the number of decibels the variable 
spectrum is above or below the standard at 
that frequency. If 350 c is chosen as the fre¬ 
quency at which to compare the standard and 
variable spectrum levels, differences of ±4 db 
were obtained for the spectra used in the test. 
The results obtained with a group of 50 listen¬ 
ers can then be plotted in the form shown in 
Figure 14. Apparently the ear can detect an 
astonishingly small shift in the frequency dis¬ 
tribution of a complex noise. 

One group of 95 listeners took both this test 
and the Seashore Pitch Test, Form A, which 
uses pure tones instead of noises. The correla¬ 
tion between the scores on the two tests was 
0.41. 

A slightly broader band of noise frequencies 
(500 to 2,000 c) was used for the loudness test. 
The results for 50 subjects are shown in Figure 
15. It will be seen from this figure that judg¬ 
ments of “louder” were easier to make than 
judgments of “softer,” and that the point of 
subjective equality (50 per cent correct judg¬ 
ments) does not coincide with the point of ob¬ 
jective equality (0-db change from standard). 
This systematic tendency to overestimate the 
loudness of the second of two stimuli has been 
attributed to a “time error” which may be 













































DIFFERENTIAL SENSITIVITY 


57 


involved in successive comparisons. A similar 
time error was observed for the Seashore Loud¬ 
ness Test, Form A, which uses pure tones. The 



Figure 14. Results obtained with pitch test for 
a group of 50 listeners. 


correlation between the two tests was 0.39 for 
a group of 99 listeners. 

3 .5.2 Application to Radio-Range Signals 

Radio-range signals are commonly produced 
by using two directional radio beams with 
crossed axes. The indicated range is the line 
along which the field strength of the two beams 
is equal. Signals are transmitted alternately in 
some sort of interlocked pattern, and the'avi¬ 
ator following the beam can recognize which 
signal is stronger. The most commonly used 
signals consist of the Morse Code letters A and 
N, interlocked in such a way that the spaces 
in one are filled by the marks of the other. When 
aural indication is used, a steady tone results 
when the plane is flying “on the beam.” 

It can be seen, therefore, that the pilot’s 
ability to fly along a radio beam will be limited 
by his ability to discriminate between the A 
and N signals. Consequently, the width of the 
range will depend, in part at least, upon the 
pilot’s differential sensitivity to intensity. 

Now, unfortunately, the definitive data on 
differential sensitivity to intensity cannot be 
directly applied here, because, whereas the lab¬ 
oratory data were obtained under quiet condi¬ 
tions, the pilot is forced to read the signal 
against a background of noise. It was neces¬ 
sary, therefore, to determine experimentally the 
differential threshold at various signal-to-noise 
ratios. 18 


A heavy, audio “static” was used to provide 
a continuous, crackling noise. The intensity 
level of the pure tone (1,000 c) was varied 



Figure 15. Results obtained with loudness test 
for a group of 50 listeners. 


between 86 and 124 db, and at each level the 
signal-to-noise ratio ranged from —10 to +50 
db. The experimental results obtained with a 
group of 10 listeners are presented in Table 1. 
The threshold is given in terms of the number 
of decibels necessary in order to hear the A 
signal 50 per cent of the time. 


Table 1. Threshold for A signal as a function of 
intensity and signal-to-noise ratio. 


S/N ratio 
in decibels 

Intensity level of tone in 
86 96 ‘ 106 116 

decibels 

121 124 

—10 

2.30 

2.23 

2.36 

2.81 



— 5 

1.46 

1.41 

1.43 

1.57 



0 

1.18 

1.07 

1.10 

1.21 

1.30 

1.40 

5 

0.86 

0.76 

0.84 

0.97 

1.05 

1.11 

10 

0.72 

0.62 

0.64 

0.74 

0.80 

0.86 

20 

0.59 

0.49 

0.46 

0.57 . 

0.65 

0.70 

30 

0.58 

0.43 

0.38 

0.41 

0.49 

0.55 

50 

0.47 

0.38 

0.32 

0.34 

0.36 

0.40 


It can be seen from inspection of these 
values that maximum sensitivity at all signal- 
to-noise ratios occurs with a signal intensity of 
roughly 96 to 106 db. The effect of adverse noise 
conditions is a radical decrease in differential 
sensitivity. From this fact it can be predicted 
that at extreme distances from the transmitter, 
where the signal is weak, the radio range may 
have four to five times the angular width which 
it has near the transmitter. Several devices 
which were developed to minimize some of the 
difficulties in receiving radio-range signals are 
discussed in Chapter 13. 


















Chapter 4 

SOME CHARACTERISTICS OF HUMAN SPEECH 


a NY investigation of a voice communication 
system must take into account the prop¬ 
erties of the speech signal which the system 
is designed to transmit. In this chapter is pre¬ 
sented a brief discussion of the important 
acoustical characteristics of speech, together 
with a discussion of certain psychophysical 
problems relating to the recognition of speech 
sounds. 


41 SOME ACOUSTICAL CHARACTERISTICS 

The production of human speech can be 
looked upon physically as a process of modula¬ 
tion. 6 A “carrier,” whose energy is supplied 
principally by the vocal cords, driven by the 
air stream from the talker’s lungs, is “modu¬ 
lated” by the speech message. This modulation 
is accomplished by appropriate movements and 
adjustments of the vocal equipment of the 
talker, notably the tongue, the lips, the teeth, 
and the cavities of the mouth, nose, and throat. 
These movements take place at the phonemic 
rate (about 10 per sec). The carrier for the 
voiced sounds is produced by vibration of the 
vocal folds. The unvoiced sounds are generated 
by breath noise resulting from forcing a stream 
of air past suitable constrictions in the vocal 
passage. In general, the frequency spectra of 
vowels consist of discrete frequency components 
with fundamentals in the neighborhood of 100 
to 200 c. The consonants, however, contain 
high-frequency components having a nearly 
continuous distribution of energy. 


The Speech Spectrum 

The frequency spectrum of connected dis¬ 
course contains not only the frequency com¬ 
ponents of the complex carriers but also the 
frequency components corresponding to the 
modulation. This includes the starting and 
stopping of sounds, changing of carriers, and 
changing of pitch. Hence, the “instantaneous” 


spectrum undergoes continuous change. The 
term spectrum which is borrowed from optics 
usually applies to steady-state phenomena. Its 
use in describing speech is justified however, 
if the contributions of the speech sounds for 
the various frequencies are suitably averaged 
over periods equal to the duration of a syllable 
or longer. The statistics of instantaneous ampli¬ 
tude vs frequency distribution is closely linked 
with the ever changing instantaneous spectrum. 
The amplitude distribution of conversational 
speech is taken up in Section 4.1.2. 

The speech spectrum is best investigated ex¬ 
perimentally by measuring the spectrum of the 
free-field sound pressure when the talker is 
located in a room free from acoustic wall re¬ 
flections (anechoic chamber). The exploring 
microphone is usually inserted into the sound 
field produced by the talker at a certain distance 
directly in front of the talker’s lips. For posi¬ 
tions along such a line extending perpendicu¬ 
larly away from the talker’s lips, the spectrum 
of speech is within wide limits independent of 
the distance from the lips and it is appropriate 
to apply the inverse square law relating inten¬ 
sity and distance. 3 The speech spectrum is then 
determined by analyzing the output voltage of 
the exploring microphone by means of a series 
of band-pass filters with contiguous pass bands. 
The output of each filter and the total, or over¬ 
all, output are measured by a suitable instru¬ 
ment possessing a square-law characteristic and 
averaged over a period of at least several sec¬ 
onds. A thermocouple and fluxmeter 4 was used 
in studies at the Bell Telephone Laboratories. 
At the Electro-Acoustic Laboratory a vacuum- 
tube circuit with parabolic characteristics was 
employed. 

An analyzing device of this type can appro¬ 
priately be called an audio-spectrometer. It is 
convenient to express the output in each band 
in terms of root-mean-square [rms] sound pres¬ 
sure per cycle in decibels relative to the stand¬ 
ard reference level 0.0002 dyne/cm 2 which is 
called the spectrum level [SL] . 3 

Figure 1 shows the average spectra measured 


58 


SOME ACOUSTICAL CHARACTERISTICS 


59 


at the Bell Telephone Laboratories’ for a crew 
of six men and five women. The results are 
expressed in terms of the free-field rms sound- 
pressure levels per cycle [SL] 30 cm directly 
in front of the talkers’ lips. Circles are drawn 
at the mid-band frequencies of the filters used. 
The speech material was obtained by reading 


aloud from English stories at what was con¬ 
sidered to be conversational speech levels. 

Figure 2 shows the average spectrum for a 
crew of seven young men as measured with the 
audio-spectrometer at the Electro-Acoustic 
Laboratory. 10 A photograph of the equipment 
is shown in Figure 3. The speech material used 



g 5 

> O 



— 

-1— 

-4- 






-o- 





= 



-L 

- 









A- 









AVERAGE 
HALF MAX 
JOE. 

F 7 YOU 
MUM IF 

AWN 

VG MEN 

ORT 

—r- 1 

. 





3 456789 2 3 456789 


80 Q * 


100 


1000 


10,000 


FREQUENCY IN CYCLES PER SECONO 


Figure 1. Average speech spectra at a distance 
of 30 cm from the talkers’ lips. (BTL) 


Figure 2. Average speech spectrum at a dis¬ 
tance of 18 in. from the talkers’ lips (Harvard) 



1. INPUT AMPLIFIER 3. VU MONITORING METER 5. NEON OVERLOAD INDICATORS 

2. BAND-PASS FILTERS 4. POWER SUPPLIES 6. SQUARE - L AW INTEGRATORS 


Figure 3. The audio-spectrometer. 




















































































































60 


SOME CHARACTERISTICS OF HUMAN SPEECH 


was the repeated test sentence: “Joe took 
father’s shoebench out; she was waiting at my 
lawn.” This sentence, devised by the Bell Tele¬ 
phone Laboratories, is said to represent a cross 
section of the most important speech sounds 



Figure 4. Variation of the speech spectrum 
with different talkers. 


used in conversational English. The speaking 
level of the talker was maintained constant at 
6 db below what each talker decided to be his 
level of “maximum (sustained) effort,” (see 
Section 4.1.3). The monitoring was accom¬ 
plished by reading the frequent peaks in the 
test sentence on a standard volume indicator 8 
(VU meter). a 



Figure 5. Average spectra for three vowels. 


good, especially since variations from speaker 
to speaker are considerable (see Figure 4). 
Variations in a given speaker from one test Jo 
the next are of secondary importance 10 as 
compared with the variations between different 
speakers. 

It is also desirable to know the spectra of 
individual speech sounds. The spoken sounds of 
English can be classified in several ways. The 
vowels can be arranged in the so-called “vowel 
triangle” on the basis of the position of the 
tongue and lips when forming the different 
sounds. 10 Figure 5 shows the long-interval rms 
sound pressures for the vowels u, a, e, averaged 
for seven men. The spectra for the vowels 6, 
a, and i, are shown in Figure 6. Figure 7 
show's the spectrum of the unvoiced fricative 
consonant sh with its predominant high-fre¬ 
quency energy. 

Although the high-frequency energy content 
of certain consonants is considerable, they tend 
to be of short duration in comparison with 
vowels, and they contribute comparatively little 
to the total energy in connected discourse. 
Although consonants are of prime importance 
for the intelligibility of speech, physical meas¬ 
urements of the type discussed do not always 
provide a suitable tool for determining the 



Figure 6. Average spectra for three vowels. 


After allowing for the different distances of 
the microphone from the talkers’ lips and con¬ 
sidering the different speaking levels, the agree¬ 
ment between Figures 1 and 2 is reasonably 


a Measurements have shown that the average of the 
deflections determined on the frequent peaks (for un¬ 
filtered speech) of a standard VU meter is about 3 db 
higher than the long-interval rms value of speech. 


presence or absence of consonants if they occur 
in combination with vowels. Figure 8 provides 
a typical example. The spectra of two words 
of the consonant-vowel-consonant type are 
compared with the spectrum of the vowel (e) 
alone. The difference between the three spectra 
is insignificant. 

These facts make it clear that an audio- 






























































































































SOME ACOUSTICAL CHARACTERISTICS 


61 


spectrometer of the type described above can¬ 
not be used for direct evaluation of the intelli¬ 
gence conveyed by speech. 15 Intelligibility 
(articulation) tests have to be resorted to for 


any given instant, however, the levels fluctuate 
widely about these average values. The sound- 
pressure distribution as a function of time and 
frequency is of importance for problems con- 




FREOUENCY IN CYCLES PER SECOND 


3 S 


£§ 

If 


Figure 7. Average spectrum for the sound “sh.” 


Figure 8. Comparison of the average spectra of 
two words and the vowel which they contain. 


this purpose. Nevertheless, information of the 
type discussed in this section is valuable in 
providing some engineering information rela¬ 
tive to the design of voice communication sys¬ 
tems and their components. 


cerning the intelligibility of speech and for 
proper design of voice communication systems. 
This distribution of speech levels was studied 
at the Bell Telephone Laboratories. 711 By means 
of a suitable audio-spectrometer and an 



Figure 9. Peak pressures in %-sec intervals 
for conversational speech at a distance of 30 cm 
from the talkers’ lips. (Average of 6 men.) 
(BTL) 



Figure 10. RMS pressures in %-sec intervals 
for conversational speech at a distance of 30 cm 
from the talkers’ lips. (Average of 6 men.) 
(BTL) 


412 The Amplitude Distribution of Speech 

The spectra of speech which have been dis¬ 
cussed in the preceding section represent the 
long-interval rms sound-pressure levels aver¬ 
aged over an appreciable period of time. At 

b Progress in this direction has been made at the Bell 
Telephone Laboratories, using a special form of visual 
presentation of the analyzed speech. 11 


arrangement of thyratron circuits and relays, 
the root-mean-square and peak sound pres¬ 
sures in intervals of Ys sec (approximate 
duration of a phoneme) were measured as a 
function of time and frequency. Figure 9 shows 
a plot of the peak sound pressures in i/a-sec 
intervals for a crew of six men as a function 
of frequency. The parameter is the percentage 
of intervals in which the pressure exceeded 

















































































































































































62 


SOME CHARACTERISTICS OF HUMAN SPEECH 


the value plotted as ordinate. For completeness, 
the level distribution for unfiltered speech is 
also given. 

Figure 10 shows a similar plot of the rms 
value in i/s-sec intervals. The long-interval 
(average) rms sound pressures for all Vs-sec 
intervals provide a basis for drawing a cumula¬ 
tive distribution curve. Figure 11 shows such 



'RMS SOUND PRESSURE LEVEL IN D8 
RELATIVE TO LONG-INTERVAL RMS VALUE 

Figure 11. Cumulative level distribution of 
conversational speech. (BTL) 

& curve for the band 1,000 to 1,400 c. The 
ordinate shows the percentage of 1/8-sec inter¬ 
vals in which the rms sound pressure exceeded 
the value given by the abscissa in terms of 
decibels above or below the long-average rms 



FREQUENCY IN CYCLES PER SECOND 

Figure 12. Peak factor of conversational speech 
at the 10 per cent level of 14-sec intervals. (BTL) 

sound pressure. From the fact that the contours 
in Figure 10 are approximately parallel, it can 
be concluded that Figure 11 is approximately 
applicable over the whole frequency range. 


It has been shown 711 that the results are not 
greatly altered if intervals of Vi-sec duration 
are used. 

If the assumption is made that intervals 
arranged in order of increasing peak pressures 
would show the same order when rms pressures 
are considered, then the peak factor (ratio of 
peak to rms pressure, in decibels) can be com¬ 
puted for any given contour in Figures 9 and 
10 and can be taken to be the peak factor of 
speech in those V6-sec intervals. In Figure 12 
the peak factor for the 10 per cent contour is 
shown as a function of frequency and also for 
unfiltered speech. Naturally, the peak factor 
for long intervals is also of interest and is 
shown in Figure 13. It means that, in unfiltered 
English speech, peaks occur about 20 db above 
the long-average rms value if the interval of 
observation is of the order of one minute. 
Clearly, information of this type is important 
for the design of communication equipment 
carrying voice signals. 


Speech at Altitude 

In the early years of World War II numerous 
complaints from aviators on high-altitude mis¬ 
sions reported a significant deterioration in 
the quality of voice signals. This problem was 
judged to be of such importance that in 1942 
a research program was undertaken at the 
Electro-Acoustic Laboratory to study quanti¬ 
tatively the effects of reduced ambient pressure 
on human talkers and listeners. A large num- 



FREOUENCY IN CYCLES PER SECOND 


Figure 13. Peak factor of conversational speech 
for long intervals. 

ber of controlled experiments were made using 
a crew of trained men in an altitude chamber. 10 
Simultaneously, controlled articulation tests 
were carried out by another group of investi- 









































































SOME ACOUSTICAL CHARACTERISTICS 


63 


gators from the Psycho-Acoustic and Electro- 
Acoustic Laboratories in high-flying airplanes 
under conditions much like those encountered 
on long-range bombing missions. This latter 
phase of the research is discussed in Chapter 9. 

In this section a brief discussion is given 
of some of the physical parameters of speech 
affected by reduced ambient pressure. The 
effects of altitude on the human voice alone is 
considered. The behavior of the voice at altitude 
when an oxygen mask is worn is discussed in 
Chapter 9. 

To determine to what extent the change in 
the talker’s voice is alone responsible for the 
altitude decrement observed in the field, data 
were taken at a simulated altitude of 35,000 
ft. e The subjects removed their oxygen masks 
temporarily and spoke into a miniature con¬ 
denser microphone 18 in. directly in front of 
their lips. The output voltage of the microphone 
was analyzed in frequency bands by means of 
the audio-spectrometer. The same procedure 
was repeated at sea level. The ratio in decibels 
of the sound pressures at sea level and at alti¬ 
tude, the altitude decrement [AD], is plotted 



100 1000 10,000 
FREQUENCY IN CYCLES PER SECOND 

Figure 14. Average altitude decrement of the 
voice alone at constant effort for various vowels. 

in Figure 14 as a function of frequency. The 
vowels u, a, e, were used in this study. The AD 
is not greatly different for the three vowels 
chosen but is positive in all three cases: the 
sound pressures at altitude are smaller than 

c By altitude is meant the so-called pressure altitude. 
In the U. S. Standard Atmosphere, for example, an 
altitude of 35,000 ft corresponds to an ambient pressure 
of 177 mm Hg. For a discussion of pressure vs density 
altitude, see Report OSRD 3106. 10 


those at sea level. Since the five speakers used 
in these experiments experienced certain diffi¬ 
culties in enunciation and timing due to the 
temporarily interrupted supply of oxygen, the 
results of Figure 14 can be regarded only as 
approximations. 

For reasons explained in Section 4.1.1 the 
altitude decrements of most consonants cannot 
be evaluated directly by an audio-spectrometer 
of conventional design. To accomplish this, 
photographs of the trace of the beam on an 
oscilloscope were made at sea level and 35,000 
ft altitude. 10 The standard test sentence, “Joe 
took father’s shoebench out; she is waiting at 
my lawn,” was used. Again the mask was 
removed during the process of speaking. Sample 
records are given for two speakers in Figures 
15 and 16. The traces represent replicas of 
the output voltage of the condenser microphone 
located in front of the talker’s lips. Since, in 
going from sea level to altitude, suitable adjust¬ 
ments in the overall amplifier gain had to be 
made, the records are best evaluated by com¬ 
paring the ratio of the deflections for a given 
consonant with the deflections of the adjacent 
vowel or vowels. Figure 17 shows the average 
gain in level for ten consonants and semivowels 
with respect to the adjacent vowels in the test 
sentence. The bars represent the averages for 
four talkers. The fricative and stop consonants 
(j, k, f, sh, ch, t) show a gain of from 10 to 
15 db. The semivowels (1, m, n, ng) show no 
appreciable net gain. The gain in level of the 
fricative and stop consonants more than offsets 
the average altitude decrement of the vowels 
(see Figure 14), and as a result consonants 
sound somewhat more distinct in speech at 
altitude. Offsetting this effect, however, is a 
“booming” phenomenon at low frequencies 
when oxygen masks are used. Most of those who 
spoke into open air at altitude noticed some 
difficulty in effecting nasal resonances and de¬ 
scribed feelings of a “stuffy nose” or “cottony 
throat.” 

In all speech measurements of the type dis¬ 
cussed, the question of “monitoring” the voice 
is important. In experiments at sea level suit¬ 
able monitoring microphones operating a 
volume indicator visible to the talker can be 
used to keep the talker’s voice “constant” from 














































SOME CHARACTERISTICS OF HUMAN SPEECH 


64 


test to test. This is not the case with measure¬ 
ments at altitude. It is pointless to attempt 
to speak at the same level at all altitudes if 


the very changes in voice quality and levels are 
themselves to be studied. A “normal” speaking 
voice could not be chosen for similar reasons. 







"JOE ' 


"TOOK" 


SPEAKER I, OPEN AIR 



p-6&SEC-H 




^V^A^A^V^/Vv^AVVvvnavvV^ 


Figure 15. High-speed photographs of speech waves at sea level and 35,000 ft. 





"bench" 

"SHE" 

35.000 FT 

pfo SEC ^1 

SPEAKER 2, OPEN AIR 







SH --*- 


E -3- 






Figure 16. High-speed photographs of speech waves at sea level and 35,000 ft. 












SOME ACOUSTICAL CHARACTERISTICS 


65 


Side-tone 1 ' monitoring was deemed undesirable 
because of its wide variability and dependence 
on the equipment used. 

A duplicable level of talking was found to 
be that of half effort. The subject was first 
required to pronounce all sounds as loudly as 



Figure 17. Average gain of consonants and 
semivowels relative to the adjacent vowels at 
35,000 ft. 


possible. He was then required to talk at what 
he considered half maximum effort. Long series 
of measurements confirmed the fact that the 
talkers make the judgment of half effort con¬ 
sistently, at both sea level and altitude. In 
terms of volume-indicator readings, the differ- 


Laboratory indicate that the average pitch at 
which a person talks does not vary appreciably 
between sea level and 40,000 ft. 

The number of words which can be said on 
one breath at a constant output level was found 
to be proportional to the air density. In normal 
speaking at 35,000 ft it is necessary to pause 
for breath two or three times as often as at 
sea level. 


Peak Factors of Vowels 

In Figure 18 the vowel sounds have been 
arranged in the “vowel triangle.” 1 The size of 
the peak factor, as measured for a sample of 
47 male talkers, is indicated by the linear 
height of the bar for each vowel. 

Since the vowels have been arranged in the 
triangle on the basis of the position of the 
tongue in the mouth in forming the vowel, those 
on the left are back vowels and those on the 



Figure 18. Peak factors of vowels for conversational and loud voices. 


ence between maximum and half effort corre¬ 
sponds to about 6 db, and this monitoring level 
was used during all the tests described above. 

Subjective estimates by listeners during sev¬ 
eral months of testing at the Electro-Acoustic 

d A method of monitoring which diverts a part of the 
outgoing speech energy (side-tone) to an earphone worn 
by the talker. 


right are front vowels. Those at the bottom 
of the triangle are formed with the tongue 
resting near the floor of the mouth. Front and 
back vowels are normally spoken with the lips 
near together, while vowels at the lower corner 
are formed with the lips apart. 

Examination of the peak factors measured 
for speech at loud and conversational levels 
























































































66 


SOME CHARACTERISTICS OF HUMAN SPEECH 


(shaded bars) reveals a curious effect. When 
the tongue is near the floor of the mouth and 
the lips are far apart, the peak factor is great¬ 
est. When the announcer is using a very loud 
voice level, however, the peak factors are 
smaller. 


42 FACTORS GOVERNING THE RECOG¬ 
NITION OF SPEECH SOUNDS 

The factors which determine whether or not 
any particular speech sound will be recognized 
on any given occasion are many and varied. 
If, however, we simplify the experimental 
situation by eliminating, i.e., averaging out, the 
different abilities and idiosyncrasies of different 
speakers and listeners 6 and by minimizing the 
distortion in the transmitting network, we find 
that some speech sounds are intrinsically more 
recognizable than others. These differences 
seem to depend upon both the physical and 
psychological nature of spoken language. 



S/N IN DB 


Figure 19. Showing the articulation of different 
classes of speech sounds as a function of the 
S/N ratio. Nonsense syllables were spoken into 
a linear system and heard against a background 
of white-noise interference in the headphones. 

Illustrative data are shown in Figure 19, 
where the percentages of correct identifications 
of different classes of speech sounds (the classi¬ 
fication used here was taken from the litera- 

e Individual differences in speakers and listeners, and 
their effects upon intelligibility are considered in detail 
in Chapter 14. 


ture) lil are plotted against the intensity ratio 
(in decibels) between the speech and a white- 
noise interference introduced electrically into 
the earphones. These data were obtained with 
a system having a relatively uniform frequency 
response. The test material was composed of 
nonsense syllables (consonant-vowel and vowel- 
consonant combinations), and in this way the 
contextual identification of the sounds was 
avoided. 

Examination of the functions reveals that 
vowel identification is more accurate than con¬ 
sonant identification. This difference is corre¬ 
lated with the greater speech power of the 
vowels. It has been estimated, for example, that 
the most powerful phoneme (“aw” as in all ) 
is 680 times (28 db) more intense than the 
weakest phoneme (“th” as in thin ) . le 

Although a small percentage of the incorrect 
identifications were simple omissions, the usual 
mistakes (at the S/N ratios used in the experi¬ 
ments) consisted of substituting some other 
speech sound in place of the sound which was 
spoken. For convenience, such incorrect identifi¬ 
cations can be called “confusions,” and it is a 
matter of some interest to discover which 
speech sounds are most easily confused. 

Analysis shows that the most frequent con¬ 
fusions of consonant sounds are between mem¬ 
bers of the same class. Thus, voiced stops are 
most often confused with other voiced stops, 
unvoiced stops with other unvoiced stops, etc. 
Less common, but of some importance, were 
confusions within the voiced sounds or the 
unvoiced sounds. Confusions of voiced with 
unvoiced consonants (and vice versa) were 
relatively rare. 

Data illustrating these relations are pre¬ 
sented in a graphic table in Figure 20. f The 
sounds spoken are grouped by classes along 
the top, and the class of the substituted sound 
is indicated vertically. The shaded area corre¬ 
sponds to the percentage of confusions of one 
class of sound with another. In Figure 20A 
the predominance of intraclass confusions is 
apparent. These data were obtained when the 

f These data were selected from a number of analyses 
made on data collected over a period of 6 months. They 
can be considered typical of the results obtained with a 
wide variety of talkers, noises, and equipments when 
the tests are conducted with meaningless syllables. 







RECOGNITION OF SPEECH SOUNDS 


67 


interfering signal (in this case, a train of 150 
“spikes” per second) was considerably weaker 
(21 db) than the speech. When the intensity 
of the interference is increased, however, such 
confusions become less predominant. Figure 
20B shows that, with a speech-to-noise ratio 


interference. 9 Thus it is possible to obtain a 
general list of easily identifiable words, but in 
order to obtain the maximum effectiveness over 
any given communication system, the vocabu¬ 
lary should be chosen for that system alone. 

Although the spectra and intensities of the 


A_ B 


S/N=+2IDB 

150 SPIKES 

PER SECOND 

VOICED 

STOPS 

« 

UNVOICED 

STOPS 

SOUND S 

VOICED 

FRICATIVES 

iPOKEN 

UNVOICED 

FRICATIVES 

SEMI¬ 

VOWELS 

TRAN- 

SITIONALS 

VOICED 

STOPS 



vVXVVVXVN- 


iti 




UNVOICED 

STOPS 






gP 

g VOICED 
O FRICATIVES 
V) 




W\\\N 




Z> ' 

Z UNVOICED 
8 FRICATIVES 


^3 


|P 




SEMI¬ 

VOWELS 




-J 



a T 

TRAN- 

SITIONALS 









S/N--3DB 

150 SPIKES 

PER SECOND 

VOICED 

STOPS 

UNVOICED 

STOPS 

SOUND : 

UNVOICED 

FRICATIVES 

SPOKEN 

UNVOICED 

FRICATIVES 

SEMI¬ 

VOWELS 

TRAN¬ 
SIT I0NALS 

VOICED 

STOPS 






TTvWTVN - 

UNVOICED 

STOPS 


imi 





CO VOICED 

O FRICATIVES 

if) 



IB 



WNV71 - 

Z UNVOICED 
8 FRICATIVES 

LVV-vM 






SEMI¬ 

VOWELS 


p 








TRAN- 

SlTIONALS 



3 

—i 




Figure 20. Illustrating the types of confusions obtained at low (plot A) and at high (plot B) intensities 
of the interfering signal. The interference in this instance was provided by a train of 150 spikes per 
second. At high intensities of the interference, the interclass confusions are increased. 


of —3 db, the percentage of confusions between 
sound classes is greater. Generalizations of this 
type proved valuable in the construction of 
special, highly intelligible vocabularies for 
military use (see Chapter 6). 

Confusion of one speech sound with another 
depends primarily upon similarities in the 
frequency spectra of certain sounds. For each 
speech sound, certain parts of the characteristic 
spectrum seem to be the critical regions with 
respect to identifiability. 1 ' 1 Consequently, the 
relation of the spectrum of a particular speech 
sound to the spectrum of the interfering noise 
and to the frequency response of the communi¬ 
cation system affects its relative identifiability. 
More attention will be paid to these interactions 
in later chapters, so it is sufficient to report here 
that ratings of word identifiability obtained on 
systems with extreme frequency and amplitude 
distortion showed significant correlations (0.23 
to 0.79) with ratings obtained using noise 


speech sounds are of fundamental importance, 
the intelligibility of speech seems to depend 
upon other factors than the physical dimensions 
of the component sounds. Against the roar of 
noise in a modern battle craft, few words are 
heard in their entirety. Rather, only certain 
sounds are recognized, and these must serve 
as cues from which the listener reconstructs 
the words spoken into the microphone. Nat¬ 
urally, the more speech sounds in a word, the 
greater the chance that enough of them will 
be recognizable for the word to be identified 
in full. It may also be said that, in general, 
the longer a word is the smaller is the number 
of other words sufficiently similar in the overall 
pattern of their speech sounds to make them 
mutually subject to confusion. To illustrate this 
relation, five groups of 20 words, having dif¬ 
ferent average numbers of sounds per word, 
were read 20 times to a group of listeners. 
The results of the experiment are shown in 

















































68 


SOME CHARACTERISTICS OF HUMAN SPEECH 


Figure 21. Similar results have been obtained 
by the Bell Telephone Laboratories. 2 

The readiness with which a word will be 
identified is clearly a function of its relation to 



AVERAGE NUMBER OF SOUNDS PER WORD 

Figure 21. Illustrating the improvement in 
articulation as the average number of speech 
sounds per word is increased. 

other words in the language. For example, the 
more frequently it is used in everyday dis¬ 
course, the greater the chances that a mutilated 
portion of the word will evoke the correct 
response from the listener. Again, the chance 
that a'word heard only in part will be rightly 


identified tends to vary inversely with the 
number of other words it resembles. For ex¬ 
ample, if the word “power,” partially masked, 
is heard only as “ower,” it will often be 
identified as “tower” or “cower” and sometimes 
as “bower,” “dour,” “lour,” “hour,” “sour,” 
or “flower.” In other words, its measured iden- 
tifiability will be lowered by the fact that many 
other words happen to differ from “power” 
only with respect to its initial and relatively 
low-powered voiceless stop, “p.” 

Finally, it is necessary to consider as “apper¬ 
ceptive variables” those elements in the experi¬ 
ence of the listeners which alter the test results. 
As apperceptive factors one must include, for 
example, the relation of the dialect of the 
listener to the dialect of the speaker. Also 
important is the educational level of the 
listener, for if the listener is unfamiliar with 
the word, he may miss it completely or confuse 
it with similar words. Related to this is the 
context, which establishes the listener’s expect¬ 
ancy. If the listener knows the stimulus word 
is a member of a set of words (for example, 
numerals or phonetic equivalents), then the 
listener will be prepared for words in that set. 

These factors, physical and psychological, 
appear to be the prime determinants of word 
identification. It would be advisable to conduct 
further experiments to evaluate the relative 
importance of each factor, in which case these 
generalizations may prove useful as a guide to 
the design of more basic experimentation. 




Chapter 5 

ARTICULATION TESTING METHODS 


A QUANTITATIVE MEASURE of the intelligi¬ 
bility of speech may be obtained by count¬ 
ing the number of discrete speech units 
correctly recognized by a listener. The pro¬ 
cedure by which this quantitative measure is 
obtained is an articulation test. Typically, an 
announcer reads lists of syllables, words, or 
sentences to a group of listeners, and the per¬ 
centage of items correctly recorded by these 
listeners is called the articulation score. This 
percentage is taken as a measure of the in¬ 
telligibility of speech. 

Articulation testing methods grew out of the 
early work of developing the telephone, and 
the Bell Telephone Laboratories were first to 
standardize the procedure as a rigorous method 
of evaluation. 2 - 3 Tests of this type have proved 
useful for comparison of communication de¬ 
vices, evaluation of the effects of noises on 
communication, measurement of the basic audi¬ 
bility of words and commands, and for rating 
and training communications personnel. 

All articulation scores are relative scores, 
contingent upon the use of specific announcers, 
microphones, amplifiers, earphones, noises, 
listeners, and test lists. Little trust can be 
placed in absolute statements about articula¬ 
tion, especially if such statements are based 
on tests with different voices and different 
listeners. In general, the only trustworthy 
statements regarding the effectiveness of com¬ 
munication systems are relative statements. 

As a consequence of the relative character of 
articulation scores, all comparisons of com¬ 
munication devices should be made with condi¬ 
tions kept as uniform as possible. A comparison 
of two microphones, for example, should be 
made only with regard to the specific conditions 
governing the particular comparison. If both 
microphones are retested with a different 
amplifier, or with different earphones, or in 
a different noise, they may well give different 
results. Similarly, in comparing listeners, it is 
important to make judgments about their rela¬ 
tive abilities to hear words or commands only 
on the basis of data obtained under comparable 
conditions. 


Some of the factors which influence articula¬ 
tion test results are important only under 
special circumstances, while others should be 
considered in every articulation experiment. 
The following list attempts to summarize the 
more obvious factors which may affect articu¬ 
lation results. 


Announcer 

Speech 

Microphone 


Amplifier 


Radio Link 


Earphones 


Earphone 

Mounting 


Listener 


1. Quality and intensity of voice. 

2. Correctness of pronunciation. 

3. Manner of holding microphone, etc. 

4. Phonetic composition and item diffi¬ 
culty. 

5. Frequency-response characteristics. 

6. Nonlinear distortion. 

7. Efficiency and impedance. 

8. Behavior at different altitudes. 

9. Directionality (shielding of micro¬ 
phone from noise — signal-to-noise 
ratio). 

10. Frequency-response characteristics. 

11. Nonlinear distortion. 

12. Input and output impedances. 

13. Gain. 

14. Peak power limitation. 

15. Shielding (noise pickup and feed¬ 
back) . 

16. Overall fidelity (response character¬ 
istic) . 

17. Signal-to-noise ratio. 

18. Loudness of side-tone channel heard 
by announcer. 

19. Overmodulation. 

20. Presence or absence of limiters. 

21. Frequency-response characteristics. 

22. Nonlinear distortion. 

23. Efficiency. 

24. Behavior at different altitudes. 

25. Design of helmet and headband. 

26. Acoustic seal at the ear (insulation 
against noise). 

27. Air volume under receiver. 

28. State of hearing (deafness). 

29. Masking of speech by noise entering 
ear. 

30. Basic ability to understand speech 
when distorted and masked. 


This list of factors is by no means exhaustive. 
It is indicative, however, of the extensive 
matrix of parameters affecting any attempt to 
evaluate in a quantitative way the relative 
merits of communication systems, the effective¬ 
ness of noises in masking speech, the audibility 
of words and commands, or the relative abilities 
of communications operators. 


69 


70 


ARTICULATION TESTING METHODS 


51 THE FORMAL ARTICULATION TEST 

Because of the complex multitude of vari¬ 
ables involved, proper experimental control of 
all the relevant conditions presents a formid¬ 
able problem. Although short cuts have been 
devised for greater economy of time and per¬ 
sonnel, the most valid and reliable results can 
be obtained only by the formal articulation 
test. The formal articulation test involves the 
use of carefully selected test materials, the 
elaborate control of individual differences in 
talkers and listeners, a relatively permanent 
testing installation, and systematic variation of 
the intensities of the speech and noise employed. 
Abbreviated test procedures necessarily slight 
one or more of these aspects. 

The following discussion summarizes the 
extensive experience of the Psycho-Acoustic 
Laboratory in using this formal method of 
testing. 5 


511 Test Material 

Since the nature of the spoken items helps 
to determine the resulting articulation scores, 
the test material must be carefully selected. For 
most testing purposes the speech sounds used 
should be representative of conversational 
speech. Furthermore, for economy of effort and 
time, it is important to group the speech units 
used for the articulation test into convenient 
test lists, each list as difficult as each other list. 
When lists of comparable difficulty are used, 
differences in articulation scores obtained with 
two different microphones may be interpreted 
as due to the differences in the instrument 
rather than to differences in the difficulty of 
the list. Since the difficulty of a test list is 
determined not only by the fundamental speech 
sounds but also by numerous psychological fac¬ 
tors, it is necessary to demonstrate by actual 
test that each test list is as difficult as each 
other list. 

The desirability of a proportional representa¬ 
tion in the test lists of the sounds which occur 
in everyday speech stems from a consideration 
of the problem of validity. A microphone which 
passes only one type of sound might test well 


with a list of words containing only such 
sounds, but the test would not be a valid indi¬ 
cation of the usefulness of this instrument for 
ordinary conversation. It is possible to state 
approximately the relative frequencies of oc¬ 
currence of sounds in “average” speech, and to 
approach these frequencies in the distribution 
of sounds among the lists of an articulation 
test. A standard reference on the distribution 
of speech sounds may be found in the litera¬ 
ture. 1 In general, as the number of different 
speech sounds in the test lists is increased and 
as the relative frequency with which each sound 
occurs approaches that found in conversational 
speech, the more valid will be the articulation 
results. 

For practical applications, it is usually de¬ 
sirable to obtain the articulation, not of the 
individual speech sound, but of combinations 
of the sounds. When combinations of speech 
sounds are employed, however, the articulation 
score depends not only upon the difficulty of the 
individual speech sounds but also upon the 
particular combinations used. In general, 
speech sounds may be combined in three differ¬ 
ent types of test items: 

1. Single syllables made up of meaningless 
combinations of speech sounds. 

2. Meaningful words given out of context as 
isolated units. 

3. Meaningful phrases or sentences in which 
there are contextual relations among the words. 

The principal differences among these three 
classes depend upon the psychological factors 
of meaning, inflection, rhythm, etc. 

When it is desirable to determine accurately 
the particular speech sounds transmitted by 
an interphone, nonsense syllables are superior 
to words or sentences as test items, because 
with this type of material, the clues otherwise 
provided by the meaning of the combination are 
minimized. The use of nonsense syllables, how¬ 
ever, requires that the testing crew be thor¬ 
oughly trained. The announcer must correctly 
pronounce the speech sounds and the listeners 
must record with phonetic symbols the sounds 
they hear. When words or sentences are used, 
on the other hand, the training program is far 
less arduous. 

The extent to which word articulation scores 




THE FORMAL ARTICULATION TEST 


71 


are influenced by meaning, inflection, etc., de¬ 
pends largely upon the particular words chosen 
for the test. Short words which provide less 
opportunity for the operation of such factors 
are usually missed more frequently than long 
words. When sentence lists are scored in terms 
of the meaning conveyed these psychological 
factors are still more important. For this 
reason, sentence articulation is typically higher 
than word or syllable articulation. Figure 1 



Figure 1. Lists of words and lists of sentences 
were compared for intelligibility under a wide 
variety of conditions. Each point on the graph 
represents 1 test of 100 words and 1 test of 50 
sentences. The curve passing through the data 
was derived from results obtained at the Bell 
Telephone Laboratories. 

illustrates the type of relation obtained experi¬ 
mentally between word and sentence articula¬ 
tion. This relation shows that sentence 
articulation is higher than the corresponding 
word articulation. 

In addition to representing conversational 
speech by lists of equal difficulty, the test items 
must be selected in such a way that the distribu¬ 
tion of item difficulty in each list will make 
possible a sensitive measuring instrument. The 
following hypothetical example illustrates the 
importance of the distribution o^pfems with 
respect to their inherent difficulty t^) the 
listener. In this example, schematized in Figure 


2, three different test lists are used in order 
to compare two interphone systems. Each of 
these test lists is comprised of twenty items 
of knoum difficulty. The items in the first list 
are uniformly distributed along the hypo¬ 
thetical scale of difficulty, as is shown in the 
figure. Ten of the items in the second list are 
difficult and ten are easy. The third test list 
includes twenty items of intermediate difficulty. 
Figure 2 also shows the articulation scores that 
would be obtained with two interphone systems, 
A and B, tested with each of the three test 
lists. 

Although the first test list would provide a 
means of distinguishing between the two com¬ 
munication systems, this list includes relatively 
few items which are heard correctly over one 
of the communication systems but missed over 
the other system. Those items which are always 
correctly recorded or always missed when read 
over the systems tested do not contribute to 
the sensitivity of the articulation test. Such 
items are “deadwood,” and if they are eliminated 
from the test list, time and effort will be saved. 

The second list, with only very hard and 
very easy items, is of little or no use in dis¬ 
tinguishing between the two systems, A and B. 
It is not an adequate test because it does not 
measure the difference in intelligibility pro¬ 
vided by the two interphone systems. It says, 
in effect, that the two systems are identical. 
Such a test list would be of value only when 
very good or very poor communication systems 
are tested. 

For the particular task of appraising these 
two interphones, the third test list, comprised 
only of items of intermediate difficulty, is the 
best list. This list is economical in that it con¬ 
tains few items of dead weight. It provides 
a sensitive measure of the relative merits of 
systems A and B. 

The foregoing example illustrates three im¬ 
portant requirements for the distribution of 
test items with respect to difficulty. If test lists 
are to be sensitive to small differences in 
intelligibility and convenient for use, the test 
items must be fairly closely distributed along 
a scale of difficulty. Secondly, those items which 
under the conditions of the tests are always 
recorded correctly or always missed are dead 




72 


ARTICULATION TESTING METHODS 


weight and may well be omitted from the test 
lists. Finally, the number of test items in a list 
should be sufficiently large and the distribution 
of difficulty sufficiently wide to embrace the 
requisite range as determined by the types of 
communication equipment under study. 


the number of tests. For this reason it is 
important to have available a large number of 
equivalent lists of test materials and to choose 
judiciously the number to be administered at 
any one time. 

As a practical consideration, it should be 



LIST I TESTED LIST 2 TESTED LIST 3 TESTED 

Figure 2. Illustrating how two communication systems would be appraised by 3 idealized lists of 20 
words each. The distribution of the items with respect to their inherent difficulty is indicated for each list. 
Each system is represented by a bar whose height, against the scale of difficulty, shows what items the 
system is able to transmit intelligibly. The resulting articulation score is shown above each bar. 


It should be pointed out that in terms of 
reliability alone, i.e., the ease with which an 
articulation score could be repeated on a second 
testing, the second list shown in Figure 2 is 
the most reliable and the third is the least 
reliable. With the second list a score of 50 per 
cent would be obtained regardless of small 
random fluctuations in the subtle factors which 
always affect articulation scores. Scores ob¬ 
tained with the third list would be quite sensi¬ 
tive to these random factors. It is plain, 
therefore, that a reliable, repeatable test is not 
necessarily a good one, and it is also obvious 
that a sensitive test is apt to be inherently 
unreliable. 

Fortunately, the reliability of an articulation 
score can always be improved by increasing 


noted that in any actual list of test items the 
distribution of difficulty would probably never 
follow the ideal scheme presented in Figure 2. 
Instead, the items would tend to follow a bell¬ 
shaped distribution along a scale of difficulty. 
For this reason, articulation tests are ordinarily 
not uniformly sensitive over the complete range 
of possible scores. The greatest sensitivity 
usually results when the testing conditions are 
so adjusted that scores near 50 per cent are 
obtained. 

The actual task of assembling lists of ma¬ 
terials for articulation testing proved to be 
both interesting and endless. Following the first 
extensive word lists compiled by the Psycho- 
Acoustic Laboratory in 1941, there was con¬ 
tinual tinkering, testing, and trying by one or 






















THE FORMAL ARTICULATION TEST 


73 


another research group in an effort to improve 
old lists or devise new ones for special appli¬ 
cations. The story of the design of nonsense 
syllable lists, monosyllabic word lists, spondaic 
word lists, and sentence lists has been discussed 
in detail elsewhere, 5 along with lists of test 
materials of these four types which were de¬ 
signed with relevant considerations in mind. 

In administering these materials, some con¬ 
sideration must be given to the use of a carrier 
sentence and to the time interval between test 
items. In word or syllable articulation tests each 
item is usually read as part of a sentence. For 
example, in the sentence, “You will write car,” 
the word car is the only word which the listener 
is required to record. 

A carrier sentence is desirable for several 
reasons. (1) The listener is prepared for the 
presentation of the test item, and variability 
in the articulation scores due to inattention or 
distraction is reduced. (2) If carbon-button 
microphones are used, the carrier phrase pre¬ 
ceding the test items serves to agitate the 
particles of carbon and reduce the variability 
inherent in such microphones. (3) The carrier 
sentence permits the announcer to modulate his 
voice so as to keep the level of his voice even 
from word to word. 

In some experiments a carrier sentence is 
selected to fulfill a more specific purpose. When 
articulation tests are conducted at high alti¬ 
tudes, for example, it may be desirable to use 
a fairly long sentence. In this way, the effect on 
articulation due to the difficulty of breathing 
at high altitudes may be assessed. One such 
carrier sentence that has been used successfully 
is “One, two, three, four, five, test item, seven.” 
All the words in this sentence, including the 
test item, should be spoken evenly and with one 
breath. No attempt should be made to accentu¬ 
ate the test item. 

For most purposes, the carrier sentence and 
not the test item should be used to monitor 
voice level. No attempt should be made to com¬ 
pensate for the typical differences in the speech 
power used in pronouncing the different sounds 
in the test item. When only the carrier sentence 
is monitored, the test item should be spoken 
with the same general effort as the rest of the 
carrier sentence. 


The time interval between test items should 
be just long enough to enable the listeners to 
record the test item. With a well-trained crew 
the average rate at which words or syllables 
may be read is one item every three to four 
seconds. With this interval it is possible to 
conduct from 25 to 35 tests of 50 words each 
in a three-hour testing session. With a session 
this long, two to four 15-minute rest periods 
should be given. 


51,2 Personnel 

The intelligibility of speech heard over a 
communication system depends upon the vocal 
quality of the announcer and the listening 
abilities of the listener, as well as upon the 
quality of the physical instruments used. Since 
large individual differences in talkers and 
listeners are to be expected, a number of factors 
should be considered in selecting the testing 
personnel. Announcers should be selected who 
are able to enunciate the fundamental speech 
sounds in a “normal” manner, where the cri¬ 
terion for normality is one of common sense. 
Listeners having the equivalent of a high school 
education and whose ages range from 17 to 30 
years are usually suitable for this work. Sex 
differences appear to be unimportant so far as 
listening ability is concerned. For most pur¬ 
poses, the listener should have reasonably 
normal hearing, as determined by standard 
audiometric procedures. 

Articulation scores obtained with inexperi¬ 
enced listeners show improvement with prac¬ 
tice. Figure 3 shows a typical learning curve 
obtained under severe acoustic stress. After 
two to three hours of practice each day for a 
period of five successive days, this crew had 
reached a relatively stable level of performance 
under the particular conditions chosen for the 
tests. 

From the thousands of articulation tests that 
have been conducted in the Psycho-Acoustic 
Laboratory with various numbers of talkers 
and listening crews, under a great variety of 
testing conditions, it is possible to make an 
approximate estimate of the optimum number 
of listeners, talkers, tests, and test words for 



74 


ARTICULATION TESTING METHODS 


the type of test situation most frequently en¬ 
countered. When the time available for testing 
is limited, it is usually most efficiently used by 
tests of about 50 words which, for purposes of 
administration, may be divided into two lists 



Figure 3. Each point on the curve represents 
the average score on 12 tests for a crew of 10 
listeners. The tests were read over an interphone 
system by three well practiced announcers. Both 
the announcer and the listeners were in an 
ambient airplane noise (115 db). 

of 25 words each. Since this length has been 
found convenient both for administration and 
scoring, the estimates of the optimum number 
of listeners, talkers, and tests are made on the 
assumption of 50-word lists. 

The score of a single listener, due to his own 
variability, may vary 7 or 8 per cent from one 
test to another under the same conditions. This 
variation can be effectively reduced by reading 
tests to a group of listeners rather than to a 
single listener and by using the mean articula¬ 
tion score of the group. The mean articulation 
score obtained from a well-trained crew of six 
to eight listeners will be sufficiently stable for 
most experiments. 

Fatigue from long hours of listening to word 
lists, particularly in the presence of intense 
noise, is a factor which might be expected to 
introduce systematic errors. However, experi¬ 
ments designed especially to measure the 
effects of fatigue on articulation scores demon¬ 
strate that under typical conditions of testing 
and in the presence of an intense ambient noise, 
little or no change in the articulation scores 
results from several hours of testing. 

The variability of the talker is a much more 
important source of instability in the final score. 
The effect of this variability can be reduced 


only by having each talker read a number of 
tests and by averaging the mean articulation 
scores for the several tests. The average of the 
mean articulation scores of a set of six 50-word 
tests read by a single talker to a crew of eight 
experienced listeners under conditions per¬ 
mitting about half the words to be heard 
correctly will probably be as stable a value as 
will be necessary for most purposes. As the 
number of tests is increased beyond six, under 
these conditions, the stability of the average 
increases only slightly with each additional test. 

Increasing the number of talkers and listen¬ 
ers has two effects: it improves reliability of 
the test and it makes the results of the test more 
representative of the results that would be 
obtained from an unlimited population. Against 
these advantages must be weighed the cost in 
time and effort. The point of diminishing re¬ 
turns is reached and passed if the numbers are 
compounded too rapidly. 

In a great many testing situations, the num¬ 
ber of different talkers used is not important. 
With some kinds of equipment, however, it is 
important to use a wide variety of talkers in 
order to obtain representative results. A man 
with a thick, fat neck may be a superior talker 
with an ordinary hand-held microphone, but he 
may be almost unintelligible over a throat 
microphone because of the padding on his 
throat. In general, it is with tests of micro¬ 
phones that the problem of selecting representa¬ 
tive talkers is most pressing. 

Variability due to inexperienced announcers 
and listeners can often be reduced by a short 
period of instruction. The way in which the 
testing personnel use the microphones and 
headsets under test are important conditions 
of an articulation test. In order to control the 
speech input to the microphone throughout any 
one test it is necessary to keep the face of the 
microphone at a constant distance from the 
lips of the announcer. Figure 4 shows the re¬ 
sults of a simple experiment illustrating the 
importance of instructing the announcer re¬ 
garding this factor. 

The instructions given to the listener regard¬ 
ing the use of headsets also depend upon the 
purpose of the experiment. When the listeners 
take the tests in an intense ambient noise, the 




THE FORMAL ARTICULATION TEST 


75 


acoustic seal afforded by the headset is an 
important factor. 

Finally, the listeners should be instructed to 
record a response for every item called by the 



DISTANCE FROM LIPS TO MICROPHONE 

Figure 4. Showing the effect on communication 
of varying the distance of the microphone from 
the lips. The tests were conducted with the an¬ 
nouncer and crew of listeners in an airplane 
noise (115 db). Each point is based upon the 
average of tests read by three announcers. 

announcer. It has been shown in many experi¬ 
ments that the listener records correctly some 
test items even though he maintains that he is 
only guessing. 


51-3 Equipment 

In comparing two devices with respect to 
the intelligibility of speech transmitted over 
them, it is important to consider the other 
parts of the communication system used for the 
tests. For example, suppose that of two micro¬ 
phones tested, one microphone does not trans¬ 
duce speech frequencies above 2,500 c and the 
other transduces all the important frequencies 
of speech. If earphones which do not transduce 
speech frequencies above 2,500 c are used for 
these tests, little or no difference would be found 
in the articulation scores provided by the two 


types of microphones. If, however, earphones 
having a wide frequency response were selected 
for this comparison, a higher articulation score 
would be obtained with the microphone which 
transduces all the important frequencies of 
speech. It may be stated as a general rule that 
the final evaluation of an instrument cannot be 
made apart from a consideration of its asso¬ 
ciated equipment. 

One of the most convenient equipment ar¬ 
rangements provides a high-fidelity system with 
sufficient flexibility to permit the substitution 
of the equipments to be tested for the corre¬ 
sponding components in the testing installation. 
Sometimes a complete system from microphone 
to headset is to be studied, and in such a case 
the testing system is not needed. More often, 
however, only one or two items of a communi¬ 
cation system are tested, and in this case the 
use of complementary equipment of good 
quality is the easiest to specify and shows the 
clearest picture of the component’s perform¬ 
ance. 

s ' 1 ' 4 Noise Conditions 

In most communication systems the ambient 
noise which reaches the ear is either picked up 
by the microphone or enters by way of acoustic 
leaks in the headphones or helmet which the 
listener wears in a noisy environment. The kind 
and the amount of ambient noise which enters 
the system through the microphone depends 
upon three main factors: the intensity and 
spectrum of the ambient noise (see Chapter 2), 
the amount of acoustic shielding which the 
construction and mounting of the microphone 
provide (see Chapter 9), and the response 
characteristic of the microphone itself. The 
acoustic seal provided at the ear by various 
headsets and helmets also varies over a wide 
range. Hence, the intensity and spectrum of 
the interfering noise leaking into the ear by 
this path may vary greatly. 

In addition to ambient noise, electrical noise 
interference is often picked up by radio equip¬ 
ment. This electrical interference may arise 
from atmospheric static, from adjacent installa¬ 
tions (notably radar), or from deliberate jam¬ 
ming attempts by the enemy. 




76 


ARTICULATION TESTING METHODS 


The noise which reaches the listener’s ears 
tends to mask portions of the speech signal. The 
amount of masking produced depends primarily 
upon the intensity and the spectrum of the 
interfering noise. By way of illustrating the 



Figure 5. Effect of loud ambient noise on the 
relation between per cent word articulation and 
received speech intensity. When the announcer 
and the listeners are in an intense ambient noise 
(120 db), the range of levels of received speech 
available for intelligible communication has been 
reduced by about 65 db. (See reference 3, p. 272.) 

effects of a particular noise, Figure 5 shows 
the extent to which an intense ambient airplane 
noise (120 db) reduces the range of levels of 
received speech available for intelligible com¬ 
munication. In order to obtain the same articu¬ 
lation score in 120 db of this noise as would 
be obtained in the quiet, it is necessary to 
increase the intensity of the speech signal 
approximately 65 db. 

The particular noise conditions used for test¬ 
ing will depend upon the purposes of the 
experiment and upon the anticipated uses of 
the equipment under test. In testing equipment 
for use in high ambient noise fields, there are 
four permutations of noise and quiet under 
which articulation tests can profitably be con¬ 
ducted : 

1. Quiet-to-quiet: both announcer and listen¬ 
ers in quiet (Q-Q). 

2. Quiet-to-noise: announcer in quiet, listen¬ 
ers in noise (Q-N). 

3. Noise-to-quiet: announcer in noise, listen¬ 
ers in quiet (N-Q). 


4. Noise-to-noise: both announcer and listen¬ 
ers in noise (N-N). 

Each of these arrangements has its analogue 
in one or another mode of field operation. 

The situation is further complicated when 
an electrical noise is added to the interference 
produced by an ambient noise. Tests conducted 
under these conditions indicate that the articu¬ 
lation scores obtained are dependent upon the 
predominant source of noise. Thus, for ex¬ 
ample, if a radio receiver is being tested for 
its susceptibility to atmospheric static and if 
an ambient noise is also used in the test, the 
ambient noise controls the articulation scores 
over the lower portion of the range of speech 
intensities. When the speech signal and static 
are well above the level of the ambient noise, 
however, the amount of interference is depend¬ 
ent upon the strength of the electrical noise. 
It is generally advisable, therefore, to simplify 
the experimental situation when the effects of 
electrical noise are under consideration by 
eliminating the use of ambient noise for acoustic 
stress. 


Speech Intensity 

Signal level is one of the most important 
determinants of the intelligibility of speech. 
For convenience, signal level in interphone com¬ 
munication may be considered a function of 
five independent variables: 

1. The voice level of the announcer. 

2. The efficiency with which the voice of the 
announcer is coupled to the microphone. 

3. The frequency response and sensitivity of 
the microphone. 

4. The gain of the associated amplifiers. 

5. The frequency response and sensitivity of 
the earphone when it is coupled to the ear. 

In experiments designed to study the rela¬ 
tion between articulation and the level of 
received speech, it is more practicable to vary 
voice level or the gain of the amplifier than 
any of the other three variables. As a matter 
of actual practice, it is amplifier gain that is 
varied in most experiments. Articulation 
measurements made under these conditions are 
commonly referred to as gain functions. In 





THE FORMAL ARTICULATION TEST 


determining gain functions, the announcer 
should maintain his speaking level constant 
throughout any one experiment. Both visual 
and auditory means of indicating voice level 
should ordinarily be provided. The output volt¬ 
age of the microphone being used may be 
employed to monitor the voice when the noise 
level due to the microphone pickup is about 5 
db or more below the speech level. When the 
speech-to-noise ratio measured in terms of the 
voltage developed by the microphone is less 
than 5 db, this method is inadequate. Further¬ 
more, carbon microphones do not provide a 
correct indication of changes in voice levels 
under typical conditions of testing. At high 



RELATIVE OUTPUT OF 633-A MICROPHONE IN OB 


85 90 95 100 105 110 115 120 125 

SOUND LEVEL OF SPEECH 3 INCHES FROM MOUTH 

Figure 6. Showing the relation between voltage 
output of various microphones as a function of 
speech input intensity. Curves represent average 
results for three speakers, each of whom read 10 
words at five voice levels when using the follow¬ 
ing types of microphones: (1) Two T-30-P and 
two T-30-V carbon throat; (2) three sonotone 
magnetic throat; (3) two T-17 (one Kellogg and 
one Universal) carbon hand-held; (4) one 633-A 
dynamic. The curves have been made to coincide 
at the lowest speech level tested. Indicated by the 
arrow is the average speech level used by a crew 
of enlisted men in flight tests at Eglin Field, 
Florida. All output voltages were measured with 
a VU meter. 


voice levels, these microphones distort the 
speech signal and an effective compression 
occurs. Consequently, changes in the output 


voltage of some, if not all, carbon microphones 
are not proportional to changes in voice level 
(see Figure 6). 

In most experiments a monitoring system 
which is independent of the system under test 
is desirable. By using a magnetic throat micro¬ 
phone with an amplifier and output meter, a 
monitoring system is obtained which is satis¬ 
factory over a wide range of experimental con¬ 
ditions. As shown in Figure 6, changes in voice 
level are quite accurately indicated by the volt¬ 
age output of a magnetic throat microphone. 
The chief deficiency of this method is the 
difficulty in placing a throat microphone in the 
same position on the throat from test to test, 
but this objection can be overcome by the 
exercise of sufficient care. 

When articulation tests are conducted at only 
one speech level it is frequently useful to know 
the sensation level of the received speech sig¬ 
nal. The sensation level of a given sound is 
the number of decibels that the sound is above 
its normal threshold of audibility. The sensa¬ 
tion level of speech may be determined by 
attenuating the speech level until a listener can 
detect the presence of about 50 per cent of the 
speech sounds without identifying any of them. 
The amount of attenuation in decibels required 
to obtain this threshold is used to determine the 
sensation level of a given sample of speech. 


516 Statistical Methods 

If a test is repeated a large number of times 
under the same conditions, scores of various 
magnitude will be obtained. A frequency dis¬ 
tribution of these scores typically shows that 
the scores occur most frequently around some 
central value, and scores which deviate mark¬ 
edly from this central value are relatively in¬ 
frequent. The dispersion of the distribution of 
average scores obtained under the “same” con¬ 
ditions will depend to some extent upon how 
well the experimental conditions have been 
controlled and also upon the number of listen¬ 
ers and the length of each test list. Under 
typical conditions of testing in the Psycho- 
Acoustic Laboratory, it has been found that 
the standard deviation (root-mean-square) of 
a distribution of articulation scores obtained 





78 


ARTICULATION TESTING METHODS 


by reading different 50-word lists under the 
“same” conditions is about five score units. 

Because of this variability among individual 
tests, it is highly desirable to base conclusions 
regarding articulation efficiency on the average 
of several repeated tests. In general, the mean 
of a group of tests is far more stable than is 
the score for a single test. This stability in¬ 
creases as the square root of the number of 
tests, so that by reading four word lists, for 
example, the expected variability of the mean 
score can be reduced by a factor of two. 

The basic principle in experimental design 
consists in controlling at known values as many 
variables as possible and in so arranging the re¬ 
maining factors that their effects are made as 
random as possible. By this procedure it is possi¬ 
ble to interpret the central or average articula¬ 
tion scores in terms of the known conditions, 
and to determine the limits of accuracy imposed 
by the errors of random sampling. The disper¬ 
sion of the articulation scores then becomes a 
measure of the precision of the experiment, 
and an estimate of the error of measurement 
may be based upon this dispersion. It is then 
a straightforward matter to assess, by conven¬ 
tional statistical procedures, the reliability of 
the differences obtained between the scores for 
different instruments, personnel, testing con¬ 
ditions, etc. Such procedures can be found in 
any standard text on statistics. 

In general, the most direct way to increase 
the precision of an experiment, in order that 
small differences in articulation may be reliably 
assessed, is to increase the number of inde¬ 
pendent articulation tests. When the amount of 
time available for testing is limited, however, 
more reliable differences will be obtained with 
short test lists than with longer test lists. Those 
errors which tend to raise or lower the average 
articulation score throughout the reading of a 
given test list will have less influence when a 
large number of short tests is used. Orders and 
arrangements can then be counterbalanced, 
since with short tests it is possible to sample 
a larger number of announcers in a given time, 
and thereby make the test more representative 
of conditions that might be encountered in 
practice. 

Finally, if a statistically significant differ¬ 


ence is not obtained in a given experiment, it 
cannot be concluded that there is no difference 
between the devices tested. An experiment in 
which more tests are conducted, or an experi¬ 
ment in which the conditions are more precisely 
controlled, may show a reliable difference 
among the communication devices compared. 


52 ALTERNATIVE METHODS OF 
EVALUATION 

The formal articulation test based on groups 
of announcers and listeners may prove too 
cumbersome and inefficient when an articula¬ 
tion study involves numerous permutations of 
experimental conditions. It is then expeditious 
to devise short-cut methods. Many schemes are, 
of course, possible and several variants have 
been tried from time to time. 


Abbreviated Testing Methods 

An interesting example of an efficient pro¬ 
cedure is the check-list method. This method 
uses one or more listeners and the talkers are 
replaced by their recorded voices. The list of 
words are recorded in several different scram¬ 
blings, and the listener follows their presenta¬ 
tion with the aid of a check list, uncovering 
each word after he hears it spoken. He then 
indicates by a check mark whether or not he 
has heard the word correctly. Since the listener 
must establish his own criterion for “hearing” 
a word, training is necessary before consistent 
results are obtained. 

The obvious disadvantage of this method is 
that the data reflect the performance of only 
one listener and are to some extent dependent 
upon his idiosyncrasies of judgment. The 
method has been used, however, with as many 
as four listeners simultaneously. Increasing the 
number of listeners does not affect the amount 
of time which can be saved by this method. 
Since the listener, instead of writing down the 
word that he hears, merely checks his correct 
responses, he is able to work at a fast pace and 
the speed-up in the articulation testing is con¬ 
siderable. 



ALTERNATIVE METHODS OF EVALUATION 


79 


If only the total scores, and not the results 
for individual items are desired, it is possible 
for the listener to follow the check list using 
a simple manual counter which he punches 
after he has decided that he heard the word 
correctly. If lists of 100 words are used, the 
counter reading can then be taken as the per¬ 
centage articulation score. 

The same considerations regarding the selec¬ 
tion of talkers and of test material apply for 
the check-list method as for the more formal 
articulation testing methods. For the purposes 
of this laboratory, 100-word lists have been 
compiled in which 15 vowels and diphthongs are 
represented by six words each. The remaining 
ten words of each hundred are used to sample 
some of the consonants. 53 Especial care must be 
used to secure high-fidelity phonographic re¬ 
cordings of the talkers’ voices. 


a ' 2 ' 2 Comparisons Based Upon Subjective 
Appraisal 

In an articulation test the valuation of the 
intelligibility of speech is based upon the num¬ 
ber of test items correctly recorded by the 
listener. In this kind of test, the listener is not 
required to appraise the quality of the speech, 
but merely to record the speech sounds that 
he hears. By contrast, methods of subjective 
appraisal require the listener to evaluate the 
quality of the speech itself. One variation on 
the subjective procedure is the method of rank 
order. This requires the listener to judge which 
of two or more samples of speech is the more 
intelligible. Another variation is the rating- 
scale method, which requires him to describe a 
sample of speech in terms of its quality. The 
listener must make this latter judgment in 
terms of a standard sample of speech provided 
for him, or in terms of his previous experience 
in evaluating speech. 

If only an approximate assessment of the 
intelligibility of a speech sample is desired, 
these subjective methods may be found useful. 
They can be employed to especial advantage 
in the preliminary evaluation of a communica¬ 
tion system. Also, the experimental evidence 
indicates that the methods may be reliably 


used to rate the intelligibility of different talk¬ 
ers (see Chapter 14) . 4 However, for the accu¬ 
rate measurement of small differences in speech 
quality, more formal articulation testing meth¬ 
ods should be employed. 


5- 2 - 3 Threshold Methods for Evaluating 
the Intelligibility of Speech 

It is frequently desirable to know the condi¬ 
tions under which speech can just be heard at 
some arbitrary level of clarity. Three of these 
threshold methods will be described: 

1. The threshold of detectability. 

2. The threshold of perceptibility. 

3. The threshold of intelligibility. 

To determine the threshold of detectability 
(sometimes called the threshold of audibility) 



Figure 7. Showing the speech-to-noise ratios 
required to obtain three types of thresholds. 
White noise (random spectrum) at various levels 
was mixed electrically with the speech. The 
abscissa level of 0 db corresponds to a sound- 
pressure level of about 108 db at the listeners’ 
ears. 

the listener Adjusts some variable (usually the 
speech level or the level of a masking noise) 
until he is just able to detect the presence of 
speech sounds about half the time. At this 




80 


ARTICULATION TESTING METHODS 


threshold level he will ordinarily be unable to 
identify any of the sounds themselves. 

The threshold of perceptibility can be used 
to determine that condition under which the 
sounds heard at the threshold of detectability 
begin to be perceived as words. In determining 
this threshold, the listener adjusts some vari¬ 
able until, in his judgment, he is just able to 
understand with considerable effort the gist of 
the connected discourse read to him. If the 
speech were made intelligible he could, with 
little or no effort, understand it, and if the 
speech were made less intelligible, he could not 
perceive a sufficient number of words to allow 
him to follow the main ideas of the passage 
read to him. 

In determining the threshold of intelligibility 
the listener adjusts some variable, until, in his 
judgment, he is just able to obtain without 


perceptible effort the meaning of almost every 
sentence and phrase of the connected discourse 
read to him. 

The results of an experiment designed to 
evaluate these three threshold methods are 
shown in Figure 7. The level of the received 
speech was held constant and the listener ad¬ 
justed the level of an interfering random noise 
until he obtained each of the thresholds. This 
procedure was repeated using two other levels 
of received speech. The results show that, for 
this type of noise, the speech-to-noise ratio 
required for the threshold of perceptibility is 
about 7 db higher than that required for the 
threshold of detectability. A further increase 
in the speech-to-noise ratio of only 4 db is re¬ 
quired to reach the threshold of intelligibility 
where the listener is able to understand easily 
every sentence. 



Chapter 6 

INTELLIGIBILITY OF SPEECH: SPECIAL VOCABULARIES 


E xperience in articulation testing quickly 
verifies the common-sense notion that some 
speech sounds, and consequently some words, 
are more communicable than others. The inten¬ 
sity and spectra of the component speech 
sounds, the number of syllables and the relation 
of a word to other words combine to produce 
wide differences in recognizability. Because of 
these differences, haphazard selection of words 
for use in the din of mechanized war will not 
result in the most effective communication. 
Consequently, the determination of the factors 
contributing to the intrinsic recognizability of 
a speech signal and the provision of vocabu¬ 
laries of words easily recognizable in noise 
became important research endeavors. 


61 METHODS OF TESTING 

It is possible to establish the differences in 
the recognizability of individual words by 
means of an adaptation of articulation testing 
methods. The procedure differs from the stand¬ 
ard articulation tests primarily in the analysis 
of the results; words are compared rather than 
communication systems. 

The series of experiments directed at this 
problem used from four to seventeen speakers 
and from eight to sixteen listeners. An attempt 
was made to select speakers representative of 
a wide variety of dialects. Different communi¬ 
cation systems were exposed to ambient and 
electrical noises and the ratings of the words 
obtained in one situation were carefully com¬ 
pared and correlated with the ratings obtained 
with different talkers, different systems, or 
different noises. 

Caution must be employed, however, in in¬ 
terpreting these results. For the practical pur¬ 
pose of selecting a highly intelligible vocabulary, 
the important point is not the absolute score 
of each word, but the comparative score of the 
word. Since in each experiment the ratio of 
signal to masking noise was set to give optimum 
discrimination among the words being tested, 


the absolute score for each word is relative to 
the signal-to-noise ratio of each separate ex¬ 
periment. In addition, even the relative stand¬ 
ings of the words tested in an experiment of 
this type must be interpreted with regard to 
the speakers and listeners participating in the 
particular experimental procedures, and finally, 
the ratings must be regarded as to some extent 
dependent upon communication systems em¬ 
ployed. If all of these factors are taken into 
account, however, it is reasonable to expect 
that words selected on the basis of objective 
tests rather than at random or according to the 
whim of radio-telephone operators can aid sub¬ 
stantially in overcoming the obstacles to suc¬ 
cessful communication in noise. 


62 ALPHABETIC EQUIVALENTS 

In oral communication of military messages, 
the “phonetic alphabet” or words used to 
identify the individual letters of the alphabet 
plays a very important part. This alphabet is 
used over the radio-telephone not only to spell 
out difficult words, but also to transmit crypto¬ 
graph messages and standard commands. It is 
essential to the success of many operations, 
therefore, that all 26 of these equivalents be 
instantly and unambiguously identified even 
against the intense noise of mechanized battle. 

Consideration of the conditions under which 
alphabetic equivalents are used in actual service 
made it apparent that a satisfactory list must 
meet the following criteria: 

1. All the words in the list should be recog¬ 
nizable when heard over an intercommunica¬ 
tion system against a background of severe 
noise. This, of course, is the most essential 
criterion. The words should be identifiable (1) 
by listeners who are comparatively unfamiliar 
with them, (2) when the words are read over a 
representative variety of communication sys¬ 
tems, (3) when the listeners represent a wide 
range of educational levels, and (4) when the 
speakers and listeners represent different re- 


81 


82 


INTELLIGIBILITY OF SPEECH: SPECIAL VOCABULARIES 


gional dialects in both the United States and 
the British Empire. 

2. All the words should be in common use, 
easy to learn, and of the utmost brevity com¬ 
patible with a high degree of recognizability. 

3. None of the words should be geographical 
or place names, since these might be miscon¬ 
strued as forming part of an operational com¬ 
mand. 

4. None of the words ought to possess a 
facetious connotation or be of negative morale 
value. For example, the British use of “nuts” 
for “N” would seem unsuitable for American 
adoption. 

In all, approximately 300 different words 
were tested, drawn from the following sources: 

1. The LCC (British) list. 

2. The USA (American) list. 

3. The CCB list, composed of words suggested 
from time to time by members of the Combined 
Communications Board. 

4. The various words used by the American 
Telephone and Telegraph Company, the Bell 
Telephone Company of New Jersey, RCA Com¬ 
munications, Western Union, and the Interna¬ 
tional Telecommunications Convention. 

5. The NDRC list, composed of the most in¬ 
telligible words suitable for use as alphabetic 
equivalents, selected from a list of 2,400 com¬ 
mon English monosyllables and dissyllables 
which had been used in articulation testing 
of communication equipment in the Psycho- 
Acoustic Laboratory over a period of eight 
months. 

In order to determine the extent to which 
these 300 words satisfied the criteria for alpha¬ 
betic equivalents, seven tests were conducted. 
In all tests, both speakers and listeners sat in 
120 db of ambient noise. The noise simulated 
the spectrum typical of an acoustically un¬ 
treated, twin-engined bomber. 

In the light of the results of these experi¬ 
ments, the following words are recommended 
as the best equivalents for each letter of the 
alphabet. Words in parentheses represent a 
second choice whidh is only slightly inferior to 
the first choice. The words in Column II com¬ 
pose the phonetic alphabet which has been 
adopted for combined United States-British 
radio-telephone procedures. 


I 

Best Equivalent Tested 

II 

Equivalent Adopted by 
U. S. and British Board 

Ablaze 

Able (Affirm) 

Boycott (Bugle) 

Baker 

Crayon (Chowder) 

Charlie 

Dinette (Dog) 

Dog 

Eardrum 

Easy 

Flower (Fortune) 

Fox 

Golden 

George 

How (Hypo, Headline) 

How 

Interrogatory (Ideal) 

Item (Interrogatory) 

Joyful 

Jig 

Knicknack (Keyhole) 

King 

Limeade 

Love 

Mary (Mike, Manhood) 

Mike 

Nowhere (Nylon) 

Nan (Negat) 

Oliver (Oatmeal) 

Oboe (Option) 

Piano (Parole) 

Peter (Prep) 

Queen (Queer) 

Queen 

Roger (Robust) 

Roger 

Sugar (Skillet, Supreme) 

Sugar 

Thyself (Trombone) 

Tare 

Umpire (Upstairs) 

Uncle 

Victory 

Victor 

William (Welfare) 

William 

X ray (Exhale) 

X ray 

Yuletide (Yellow) 

Yoke 

Zanzibar (Zebra, Zodiac) 

Zebra 


On the basis of the articulation test results, 
the adopted list is not, in various instances, 
the best list available. It must be remembered, 
however, that a number of factors other than 
recognizability must be taken into account in 
composing any set of military signal pro¬ 
cedures. 

63 ALTERNATIVE PRONUNCIATIONS 
OF NUMERALS 

Numerals, like the phonetic alphabet, are 
such frequent and crucial components of oral 
messages that their instant recognition is es¬ 
sential to the success of many military opera¬ 
tions. As normally pronounced, however, cer¬ 
tain numerals are often confused with other 
numerals and words, even in the quiet. Against 
the intense noises of war such confusions in¬ 
crease, and some numerals become almost 
inaudible. To overcome these defects, various 
modified pronunciations of numerals have been 
proposed. The laboratory’s problem was, there¬ 
fore, to evaluate experimentally the compara¬ 
tive identifiability in noise of the various al- 







VOCABULARIES OF HIGHLY INTELLIGIBLE WORDS 


83 


ternative pronunciations of numerals. The 
pronunciations tested were drawn from the 
current United States and British pronuncia¬ 
tions, the pronunciations employed by the Bell 
Telephone Company, and several other pro¬ 
nunciations proposed by the Psycho-Acoustic 
Laboratory. 

Three experiments were conducted in order 
to establish a list of pronunciations for the 
single digit numerals in which, for U.S. and 
British speakers and listeners using a variety 
of local dialects, each pronunciation is the most 
identifiable of the various alternatives, and no 
pronunciation is readily confused with any 
other numeral in the list. 

Both on the grounds of superior identifia- 
bility and lack of susceptibility to confusion, 
the following recommendations were made: 


No. 

Recom¬ 

mended 

Acceptable 

Not 

Acceptable 

0 

Ze-ro 

Zero 

Oh 

1 

Wun 

Uh-wun 

Wun-er 

2 

Too 

Tuh-hoo 


3 

Thuh-ree 


Th-r-eee; Three 

4 

Four 

Fo-wer 


5 

Fi-i-v 

Five 

Fi-yiv; Fife 

6 

Six 


See-yix; Sixer 

7 

Seven 

Sev-ven 


8 

Ate 


Ate-er 

9 

Niner 


Ni-yen; Nine 


Since these experiments were conducted and 
reported to the Services, the following pronunci¬ 
ations of numerals have been adopted for com¬ 
bined United States - British radio-telephone 


procedures: 


Zero 

Fi-yiv 

Wun 

Six 

Too 

Sev-ven 

Thuh-ree 

Ate 

Fo-wer 

Niner 


With the exception of “fi-yiv,” all these 
choices proved satisfactory in the audibility 
tests. Since “ni-yen,” which had a strong tend¬ 
ency to be confused with “fi-yiv,” has been 
changed to “niner,” it may well turn out that 
the audibility of “fi-yiv” will be greatly im¬ 
proved. 


6 4 TELEPHONE DIRECTORY NAMES 

Telephone directory names make up another 
important component of standard military com¬ 
munication procedures. These are words used 
to identify telephone units during the course 
of a military operation. In the summer of 1942, 
a representative of the Office of the Chief Sig¬ 
nal Officer requested the Psycho-Acoustic Lab¬ 
oratory to establish experimentally the com¬ 
parative intelligibility in noise of the 500 
five-letter telephone directory names used by 
the U. S. Signal Corps. 

The telephone names submitted for tests had 
not been devised on the basis of intelligibility 
in noise, and it turned out that the different 
letters of the alphabet had varying numbers 
of satisfactorily identifiable names. Under each 
letter of the alphabet, even those words which 
could be regarded as satisfactory exhibit great 
diversity in recognizability. Also, since no name 
on the list was more than five letters in length, 
a great many of the most audible words in the 
language did not occur at all. 

It would be possible, of course, to arrange the 
call names in order of identifiability and to 
devise directions for their use which insured 
the choice of the most audible words. The 
Psycho-Acoustic Laboratory has prepared such 
a listing with suggested instructions for its 
use. 2a Even so, the list is imperfect and the 
instructions concerning its use are unneces¬ 
sarily complicated and cumbersome. Conse¬ 
quently, it was suggested that the telephone 
directory names be either supplemented or 
replaced by words from a uniform list of 1,000 
highly intelligible words described below. 


VOCABULARIES OF HIGHLY 
INTELLIGIBLE WORDS 

Each of the experiments discussed so far was 
intended to supply a small stock of speech sig¬ 
nals for a single, specialized requirement. In 
the course of these experiments, it became evi¬ 
dent that it would save considerable time and 
effort if a large scale experiment were to be 
conducted to provide an extensive general vo- 







84 


INTELLIGIBILITY OF SPEECH: SPECIAL VOCABULARIES 


cabulary of highly intelligible words which 
could be drawn upon by all branches of the 
Armed Services to satisfy their special needs. 
From this vocabulary any branch of the Armed 
Services could select sets of speech signals to be 
used for military codes; for telephone directory 
names; for names of ships, planes, tanks, and 
infantry units; for standard operational com¬ 
mands; and for any other needs which might 
arise in the course of military communications. 
The allocation of words drawn from this vo¬ 
cabulary might replace the common practice of 
using speech signals devised more or less hap¬ 
hazardly without knowledge of their intrinsic 
intelligibility. 

Experience in the field and laboratory has 
shown that such a vocabulary must satisfy at 
least three criteria. 

1. All words must be easily identified in 
noise. 

2. The words should not conflict with speech 
signals already in use for other purposes. 

3. There should be a large number of names 
of commonly recognized classes and subclasses 
of objects (animals, plants, minerals, colors, 
etc.) to identify the interrelations among mili¬ 
tary units. 

In order to provide a final list of 1,000 words, 
several thousand words were reviewed on the 
basis of evidence collected in previous experi¬ 
ments and then reduced to the final testing list 
of 1,600 words. Half these 1,600 words were 
words that had turned out to be the best in a 
list of the 3,000 most common monosyllables 
and dissyllables in the language; 400 were tri¬ 
syllables and quadrisyllables selected from 
Webster’s Collegiate Dictionary, and 400 were 
names of commonly recognized classes and sub¬ 
classes of objects (animals, minerals, plants, 
colors, musical instruments, presidents, col¬ 
leges, stars, Indian tribes, textiles, characters 
and books of the Bible). 

The results of the tests and the recommended 
list of 1,000 words have been reported in detail 
elsewhere. 2 The vocabulary recommended by 
the Psycho-Acoustic Laboratory was later 
adopted by the War Department and reprinted 
in a War Department technical bulletin. 4 A 
large proportion of the recommended words are 


trisyllables because intelligibility tends to in¬ 
crease as the number of syllables in the word is 
increased. Unless there is a pressing need for 
economy of utterance, it is necessary to use a 
large proportion of trisyllables in order to 
make efficient communication possible in the 
most severe conditions of noise. 

Upon conclusion of this research a request 
was received from the Signal Corps to supply 
10,000 highly audible tactical call signs for 
use as call names for telephone units. Unfortu¬ 
nately the number of English words which are 
nongeographical in denotation, familiar to the 
average fighting man and also identifiable in 
noise, was exhausted by a list of 3,120 call 
signs. 1 This figure includes the 1,000 highly 
intelligible words discussed. Despite the un¬ 
paralleled richness of the English language, it 
falls far short of providing 10,000 single words 
satisfying the criteria for military call signs. 

To meet this deficiency, therefore, it is possi¬ 
ble to devise a list of two-word combinations 
all of which will be both highly intelligible in 
noise and within the working vocabulary of the 
enlisted man. It is necessary to select the fol¬ 
lowing from the experimentally tested vocabu¬ 
lary of 1,000 words: (1) 80 “index words” 
which are the initial elements in the combina¬ 
tion call signs and (2) 500 highly intelligible 
“base words” to serve as the second element 
in the combination call signs. 

By combining these index and base words, it 
is possible to obtain up to 40,000 separate 
highly intelligible two-word phrases. From 
these, the 10,000 combinations which are most 
satisfactory can be chosen for use as tactical 
call signs. 

Such a list of combination call signs has the 
disadvantage that each signal is three or more 
syllables in length. Also, some of the phrases 
border on the nonsensical. It is doubted, how¬ 
ever, that the latter is a real objection to their 
use. Indeed, in view of the American soldier’s 
irrepressible love for the sassy and ridiculous, 
the sometimes unusual and amusing products 
of random combination may be more of a 
stimulus than a detriment to morale. Few en¬ 
listed men fail to derive enjoyment from having 
their telephone units identified by such names 



VOCABULARIES OF HIGHLY INTELLIGIBLE WORDS 


85 


as “Noah’s Schooner,” “Joshua’s Woodpecker,” from the final list of 10,000 call signs. There 
or “Cornell Apple-jack.” The large stock of seems to be no way, however, in which anything 
words available makes it easily possible to else but word combinations can yield the nec- 
eliminate any truly objectionable combinations essary number of signals. 



Chapter 7 

INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


V OICE communication equipment used by 
the Armed Services was improved mark¬ 
edly during the course of World War II, espe¬ 
cially as regards the reduction of frequency 
distortion. Headphones providing a uniform 
frequency response up to 4,000 c replaced 
resonant “peaked” phones. The frequency- 
response characteristics of microphones, inter¬ 
phones, and radios were also improved. 

Although the problem of frequency distortion 
was more vigorously attacked, amplitude dis¬ 
tortion was hardly less prevalent in Service 
equipment. Amplitude distortion is especially 
marked in military communication because the 
din of battle and the noise of fighting machines 
require unusually intense speech. Loud speech 
drives the communication circuits harder than 
would conversational speech, and amplitude 
distortion often results. 

The question inevitably arises, therefore, as 
to what kind and how much distortion can be 
tolerated without seriously interfering with 
the intelligibility of speech. The intelligibility 
of speech transmitted by resonant systems and 
the effects of amplitude distortion (overload¬ 
ing) were first studied at the Bell Telephone 
Laboratories, 1 and the purpose of the experi¬ 
ments discussed herein was to supplement and 
extend these early observations. 


71 AMPLITUDE DISTORTION 6 

Types of Amplitude Distortion 

Amplitude (or nonlinear) distortion results 
whenever a signal is passed through a non¬ 
linear circuit. For purposes of rough classifi¬ 
cation, such circuits might be thought of as 
falling into three categories: 

1. Circuits which transmit low-amplitude 
parts of the wave more efficiently than high- 
amplitude parts. This is by far the most fre¬ 
quent type of distortion. 

2. Circuits which transmit high-amplitude 
parts of the wave more efficiently than low- 
amplitude parts. 


3. Circuits which transmit parts of the wave 
on one side of the time axis more efficiently than 
parts of the wave on the other side of the 
time axis. 

To investigate experimentally the effect of 
these types of distortion on the intelligibility 
of speech, it was decided to employ representa¬ 
tives of each of these types of amplitude dis¬ 
tortion. Consequently, vacuum-tube circuits 
were arranged to simulate various amounts of 
amplitude distortion introduced by: 


UJ 

o 

<1 


GRADUAL 

SYMMETRICAL PEAK CUPPING 



INPUT VOLTAGE 


Figure 1 . Gradual, symmetrical peak clipper. 
The oscillograms superimposed on the coordinates 
show instantaneous input-output characteristics 
of a nonlinear circuit which could be adjusted for 
linear response, for gradual peak clipping, or for 
sharp peak clipping. 


la. A push-pull amplifier with a gradual 
overload characteristic. For convenience, this 
is referred to as gradual, symmetrical peak 
clipping (see Figure 1). 


86 













AMPLITUDE DISTORTION 


8' 


lb. A push-pull amplifier which overloads 
abruptly as the input signal is increased past a 
critical point. This type of distortion is referred 
to as sharp, symmetrical peak clipping (see 
Figure 2). 


2. A misaligned Class B amplifier which dis¬ 
criminates against weak signals. Such distor¬ 
tion is called center clipping (see Figure 4). 

3. A misaligned Class B amplifier which am¬ 
plifies positive voltage swings less than negative 


SHARP 

SYMMETRICAL PEAK CLIPPING 



-0 + 

INSTANTANEOUS 
INPUT VOLTAGE 


Figure 2. Symmetrical peak clipper. The 
oscillograms show input-output characteristics 
and wave forms for the sharp symmetrical peak 
clipping circuit. 

lc. An unbalanced amplifier which overloads 
abruptly on swings in one direction only. This 
is called a sharp, asymmetrical peak clipping 
(see Figure 3). 


SHARP 

ASYMMETRICAL PEAK CLIPPING 


UJ 

o 

< 



INPUT VOLTAGE 


Figure 3. Asymmetrical peak clipper. In these 
oscillograms clipping occurs on only one side of 
the time axis. 


voltage swings. This is called “linear rectifica¬ 
tion” (see Figure 5). 

The nonlinearity of such circuits is best de¬ 
scribed by the input-output characteristics. Two 
input-output characteristics can be distin¬ 
guished: (1) The functional relation between 



















88 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


the effective (rms) values of input and output output characteristics of these circuits are 
voltages, or (2) the functional relation between shown in Figures 1 through 5. The input waves 
the instantaneous values of input and output at the bottom, reflected in the input-output 
voltages. In some circuits, such as “compress- characteristics, result in the output waves at 
ing” or “expanding” amplifiers, the effective the right. 


V) 

z 


CENTER CUPPING 



INSTANTANEOUS 
INPUT VOLTAGE 


"LINEAR rectification" 



INSTANTANEOUS 
INPUT VOLTAGE 


Figure 4. Center clipping. The center part of 
the wave, nearest the time axis, is eliminated and 
only the peaks of the wave are passed. 


Figure 5. “Linear rectification.” The distortion 
depends upon the angle 8 of the right-hand leg 
of the input-output characteristic. The third 
characteristic represents half-wave rectification. 


characteristic is nonlinear, but the instantane- ™ Measurements of Amplitude Distortion 
ous characteristic is essentially linear. For the 

circuits listed above, however, both the effective A particular nonlinear circuit does not nec- 
and the instantaneous input-output character- essarily produce similar distortions in dissimilar 
istics are nonlinear. The instantaneous input- signals. Figure 6 illustrates this point with 

























AMPLITUDE DISTORTION 


89 


three cases. It is evident from the illustration 
that the output signal which results when a 
signal is passed through a nonlinear circuit is 
determined both by the characteristics of the 
signal and by the characteristics of the circuit. 



rn 


rr 


ri\, 

J 


tT7 

m 

p 

Li j 

\j 


-o+ -o+ -o + 

input input input 

Figure 6. Illustrating how the same input-out- 
put characteristic may affect different wave 
forms. 

For this reason it is desirable to measure the 
distortion of the circuit in question with a sig¬ 
nal of the type which the circuit is normally 
called upon to handle. 

Because speech is too complex a signal for 


PORTION CLIPPED FROM 



Figure 7. Showing the effect of a clipping 
circuit on a speech wave. The distortion produced 
can be expressed in terms of the proportional 
reduction in the amplitude. 

simple analytical procedures, however, it is not 
generally practicable to measure distortion with 
speech as the test signal. The simplest proce¬ 
dure for handling this problem is to specify 
the instantaneous input-output characteristics 


of the distorting circuit and the intensity of 
the speech signal at the input to the circuit. 
For indicating distortion, the most appropriate 
measure of speech intensity is the average peak 
amplitude of the items in the speech sample. 
Thus specified, the distortion of the speech wave 
can then be described verbally as, for example, 
“sharp, symmetrical peak clipping at one-half 
peak amplitude,” or “6-db sharp, symmetrical 
peak clipping.” Figure 7 illustrates the meaning 
of these statements. 

In distorting a sinusoidal signal, a nonlinear 
circuit has the effect of producing other fre¬ 
quencies which are harmonics of the input fre¬ 
quency. The power (or voltage) in these har¬ 
monics, when stated as a percentage of the 
power (or voltage) in the fundamental or in 
the fundamental plus harmonics, can be used as 
an expression of the degree of distortion. Al¬ 
though the resemblance between speech and 
sinusoidal waves is remote, several such expres¬ 
sions are in common use and lead to the defini¬ 
tion of the term per cent harmonic distortion 
[HD]. 

Inasmuch as speech is itself comprised of 
complex waves, complex signals are of interest 
in expressing distortion. In transmitting a com¬ 
plex wave, a nonlinear circuit may give rise to 
modulation products or “combination frequen¬ 
cies” equal to sums of or differences between 
component frequencies of the input signal or 
their harmonics. An input signal consisting of 
two frequency components equally intense and 
harmonically unrelated can, therefore, be used 
to measure so-called per cent combination-tone 
distortion [CTD]. For this expression of dis¬ 
tortion, the harmonics of the two input fre¬ 
quencies are not included as combination tones. 

It is especially important that HD or CTD 
introduced by speech-transmission circuits be 
measured with test signals of the same peak 
amplitude as the speech they are normally 
called upon to pass. If the speech is equated 
to a sine wave on the basis of effective or aver¬ 
age voltage, the high peaks of the speech wave 
may be distorted by a circuit that would pass 
sine waves unimpaired. 

In Figure 8 typical expressions of HD and 
CTD are compared for two types of clipping. 
In the following sections distortion is discussed 



























90 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


principally in terms which indicate the amount 
of distortion with direct reference to the char¬ 
acteristics of the nonlinear circuit. In addition, 
however, results have been plotted in terms of 




/ <t 

/ 



/*; 

r j?/ 




/ 


( 

/ 

</ 


CIO 

GL'P^ 

-r 

// 

f 


Hr 


[HO 



°0 6 12 18 24 

CLIPPING IN DB 


Figure 8. Harmonic and combination-tone dis¬ 
tortions are shown as functions of two types of 
clipping. 

HD and CTD. Of these two expressions, the 
latter seems to bear a somewhat more direct 
relation to the effect of distortion on intelli¬ 
gibility. 


713 Effects of Amplitude Distortion on 
Intelligibility 

The effect of each type of distortion on the 
intelligibility of speech was determined by 
means of word articulation tests (see Chapter 
5). A nonlinear circuit was introduced into an 
otherwise linear, wide-band communication sys¬ 
tem, and the efficiency of communication over 
the system was measured. Tests were conducted 
in quiet and with several types and levels of 
noise. 

Peak Clipping 

Results with peak-clipping circuits confirm 
the tests at the Bell Telephone Laboratories in 
indicating that severe overload distortion can 
be tolerated without appreciable impairment of 
intelligibility. The amount which can be toler¬ 
ated depends upon noise conditions and upon 
the intensity of the speech signal with respect 
to the listener’s threshold of hearing. Under 


any but the most extreme circumstances, how¬ 
ever, the speech wave (as viewed on an oscillo¬ 
scope) can be stripped down to one-quarter 
amplitude (12 db of sharp, symmetrical peak 
clipping) without any noticeable effect upon 
the intelligibility of speech. As a matter of 
fact, the intelligibility of speech is not lost 
entirely even when the speech wave is clipped 
and reamplified in such a way as to produce 
nothing but a succession of “square” waves. 
Expressed in decibels, this approximates in¬ 
finite peak clipping. 

A comparison of the intelligibility of peak- 
clipped with undistorted speech must, however, 
specify the methods of measuring the two sig¬ 
nals. If the clipped and unclipped signals are 
compared for intelligibility at equal peak volt¬ 
ages, the clipped signal is found to be more 
intelligible. When they are compared at the 
same effective or average voltage levels, the 
intelligibility is approximately the same. And 
if the two signals are equated on the basis of 
the voltage which the clipped signal would have 
if it were not clipped, the unclipped speech is 
more intelligible. 

For example, suppose that a typical speech 
sample is used to compare 0-db with 24-db peak 
clipping. With previously undistorted speech, 
the peak voltage (read on a cathode-ray oscillo¬ 
scope) will be about 12 db higher than the 
average voltage (read on a VU meter) for 
individual words. Peak clipping of 24 db (as¬ 
suming no other voltage gain or loss in the 
nonlinear circuit) lowers the peak voltage 24 db 
and also lowers the average voltage approxi¬ 
mately 14 db. The output signal now has a peak 
voltage only 2 db higher than its average 
voltage. 

If this signal is now compared with the un¬ 
dipped signal for intelligibility, the experi¬ 
mental finding is that the clipped speech must 
be amplified approximately 14 db in order to 
give the same articulation score, i.e., when the 
average voltages of clipped and unclipped 
speech waves are equal, their intelligibility is 
approximately the same (up to at least 24-db 
peak clipping). Thus, 14 db are “lost” in the 
sense that 14 db more amplification must be 
provided. Given the same average voltages, 
however, practically nothing is lost or gained 

















AMPLITUDE DISTORTION 


91 


by clipping. But in terms of peak voltages, the 
unclipped speech will give the same articulation 
score with a peak voltage 10 db lower than the 
peaks of unclipped speech, so 10 db have been 
“gained.” 

Thus, it is seen that the effects of peak clip¬ 
ping may prove a handicap in some situations, 
but in others it may become a benefit. If the 
gain of the nonlinear circuit (defined as the 
gain for unclipped signals) is constant, but 
the clipping level is variable, clipping will lower 
intelligibility. When the clipping level is con¬ 
stant, however, and the gain is variable, clip¬ 
ping improves intelligibility. It is fortunate 


are shown in Figures 9, 10, and 11. These three 
figures are drawn from the same data and rep¬ 
resent the three ways of expressing the effects 
of peak clipping discussed above. The random- 
noise interference had a spectrum of constant 
energy per octave band, and the two levels of 
noise used were equivalent to 90 and 120 db 
re 0.0002 dyne/cm 2 measured in a 6-cu cm 
coupler. In Figure 9 the data are plotted in 
terms of the gain in the speech channel and 
demonstrate that more amplification is required 
for peak-clipped speech to attain the same ar¬ 
ticulation score. Figure 10 presents these data 
in terms of the average level of the speech 



CAIN IN 03 


Figure 9. Functions relating articulation to gain with peak clipping and background noise as parameters. 


that in the majority of communication situa¬ 
tions the latter is more nearly the case than the 
former. 

Thus far we have considered only the case of 
peak-clipping speech heard in quiet. The effect 
of adding noise depends upon whether the 
speech and noise are mixed before or after the 
distortion is introduced. If the noise is added 
after the speech has been distorted, the effect is 
to raise the listener’s threshold of hearing 
(masking) but otherwise the statements above 
are still applicable. The only exception occurs 
when the noise is so intense that the speech 
intensity reaches the ear’s threshold of feeling. 
Under this condition, clipping the speech peaks 
acts to protect the ear, and greater intelli¬ 
gibility can be obtained. 

Data supporting these general conclusions 


signal received at the listener’s headphones 
(as indicated by a VU meter) and shows that 
articulation is relatively independent of peak 
clipping when the same average level is main¬ 
tained. Figure 11 shows these data when the 
speech level is taken as the peak amplitude of 
the speech signal which reaches the listener’s 
ears. These curves demonstrate that when peak 
values are the limiting factor the intelligibility 
of speech is improved by clipping the speech 
peaks as much as 24 db. 

When the speech and noise are mixed before 
the peak clipping occurs, then both speech and 
noise may be distorted by the nonlinear circuit. 
Cross modulation between the two signals would 
be expected to result in more severe impairment 
of intelligibility than was found in the experi¬ 
ment above. 












92 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


One practical situation occurs when an un¬ 
shielded microphone is used in intense ambient 
noise. The detrimental effects of peak clipping 
are slightly more pronounced when the articu¬ 
lation tests are conducted with the announcer 


increases. Speech from a “close-talking” micro¬ 
phone (MC-253, magnetic), which picks up 
relatively little ambient noise, suffers less im¬ 
pairment from peak clipping than does speech 
from a standard condenser microphone (WE 



Figure 10. Functions relating articulation to level of received speech with peak clipping and background 
noise as parameters. 



Figure 11. Functions relating articulation to peak amplitude of received speech with peak clipping and 
hackgi - ound noise as parameters. 


and listeners in an intense ambient noise field. 
This result was obtained for various ambient 
noises and types of amplitude distortion. Simi¬ 
larly, the detrimental effect increases as the 
amount of noise picked up by the microphone 


640-A) which has no provision for noise ex¬ 
clusion. This result is shown in Figure 12. Tests 
with a throat microphone (T-30-P) also re¬ 
vealed little impairment in intelligibility with 
peak clipping. In spite of the poor quality of 













AMPLITUDE DISTORTION 


93 


throat-microphone speech, the throat micro¬ 
phone picks up relatively little noise to be inter- 
modulated with the speech, and thus proves 
relatively resistant to the effects of such dis¬ 
tortion. 

HD 

PERCENT HARMONIC DISTORTION 



Figure 12. Articulation as a function of peak 
clipping. Talker and listeners were located in 
random noise field of 110 db. Filled and unfilled 
circles distinguish two talkers. 

More complete data relative to this problem 
were obtained by the check-list testing method. 
The results are shown in Figure 13. The amount 
of peak clipping is here specified by measure¬ 
ments made with speech alone. The spectrum 
of the unclipped noise was flat by octaves. The 
noise level when not clipped corresponded to 
100 db re 0.0002 dyne/cm 2 in a 6-cu cm coupler. 
The intensity of this noise was held constant 
at the input terminals of the clipping circuit. 
Since speech and noise together reach peak 
amplitudes higher than speech alone, it is nec¬ 
essary to distinguish between “no clipping” of 
the combined signals and clipping at the aver¬ 
age peak amplitude of the speech alone (0-db 
clipping). 

The several amounts of clipping yield a family 
of curves separated by only about 5 db. Com¬ 
parison of Figure 13 with Figure 9 shows that 
peak clipping causes less impairment of intelli¬ 
gibility when noise is mixed with the speech 
before distortion. This is due to the fact that 
clipping the peaks reduces the average noise 


voltage about as much as it reduces the average 
speech voltage. Consequently the signal-to-noise 
ratio is decreased less when the combined signal 
is clipped than it is when the speech alone is 
clipped and then mixed with the noise. 

This comparison does not, however, present 
the complete picture. When speech was clipped 
before being mixed with noise, it was seen that 
the loss in intelligibility could be made up by 
additional amplification. When the speech and 
noise are mixed before clipping occurs, how¬ 
ever, any subsequent amplification will change 
both speech and noise, thus leaving the speech- 



60 70 80 90 100 110 120 

GAIN OF SPEECH CHANNEL IN DB 
oob • threshold of audibility for undistorted speech in quiet 


Figure 13. The curves show the relation be¬ 
tween articulation and the gain of the speech 
channel when a constant amount of noise was 
mixed with the speech before the signal was 
clipped. The amount of clipping indicated for 
each curve was determined in the absence of 
noise. 

to-noise ratio unaffected. For this reason it 
must be concluded that, whenever possible, the 
signal should be kept free from noise until it 
has been clipped. 

A second practical case where speech and 
noise may be mixed before they are peak 
clipped occurs in radio-telephone communica¬ 
tions (see Chapter 13). So-called “noise-peak- 
limiter” circuits are used in radio receivers 
to minimize pulse-type interference and to im¬ 
prove intelligibility. Some of these limiters are 
essentially peak clippers. The results just de¬ 
scribed show that impairment results from clip- 
ing a mixed signal if the noise has strong 














94 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


low-frequency components and a relatively low 
peak factor. It is of interest, therefore, to 
examine results for intermittent, high-peaked 
interference of the type frequently encountered 
in radio communication. 

Tests were conducted as described above, but 
with audio static as interference. Because the 
static spikes were of extremely high amplitude, 
it was necessary to clip at amplitude levels 
corresponding to twice and four times the 
average peak amplitude of the test words. For 
convenience, these additional clipping condi¬ 
tions are designated as —6 db and —12 db, 
respectively. 



Figure 14. Effects of peak clipping when static 
passes with speech through nonlinear circuit. The 
levels of the speech signal are expressed in 
decibels above the threshold for undistorted 
speech in quiet. The negative values of peak 
clipping affected the static but not the speech. 

In Figure 14 the articulation results are 
plotted against peak clipping. A curve is shown 
for each level of received speech at which the 
tests were conducted. Since the static was 
maintained at a constant level, the increasing 
levels of received speech correspond to in¬ 
creasing speech-to-static ratios. 

With the particular type and level of static 
used, the optimal amounts of clipping lie be¬ 
tween 6 db (for higher levels) and —6 db 
(for lower levels). The amount of improvement 
in intelligibility under the most effective condi¬ 
tions is between 10 and 20 articulation points. 
These data indicate, therefore, that peak 
clipping at the proper amplitude level can 


improve the intelligibility of speech in the 
presence of static. 

Subjective observations of the effects of peak 
clipping on the quality of the speech indicate 
that the quality is impaired surprisingly little. 
On the contrary, with 12-db peak clipping it 
sounds as though the speaker were enunciating 
with special care. Although small amounts of 
peak clipping are not objectionable, tests with 
speech signals have shown that the point at 
which clipping begins can be detected almost 
as accurately by ear as by examination of the 
trace on a cathode-ray oscilloscope. The sensi¬ 
tivity of the ear to these slight changes in 
harmonic content is surprisingly acute (see 
Section 3.5.1). The visual determination is to 
be preferred, however, because of its greater 
reliability. 

Speech is still quite intelligible with 24-db 
peak clipping, but sounds unnatural and 
“grainy.” In general, the intensity of the peak- 
clipped speech is monotonously uniform, and 
this is attributable to the fact that the clipper 
circuit reduces the level of the more intense 
words but leaves the weaker words relatively 
unaffected. 

Center Clipping 

The extreme tolerance of the ear for peak 
clipping is matched by an equally marked in¬ 
tolerance for center clipping. The results of 
center-clipping tests are shown in Figure 15. 
The comparable curve for peak clipping is 
replotted from Figure 12. More than 1-db 
center clipping severely reduces the articulation 
scores. 

An explanation of the highly dissimilar 
effects of peak and center clipping can be given 
in terms of certain characteristics of a typical 
speech wave. The intelligibility of speech de¬ 
pends primarily upon the correct identification 
of consonant sounds, while the recognition of 
vowels is much less critical. The nature of 
speech is such, however, that the less important 
vowel sounds are about 10 to 15 db more 
intense than the very important consonant 
sounds. Consequently, when speech is peak 
clipped it is primarily the vowels which are 
affected; the consonants are increased in rela¬ 
tive amplitude, and the speech remains in- 











AMPLITUDE DISTORTION 


95 


telligible. With center clipping, on the other 
hand, the weak but important consonant sounds 
are proportionately more affected than the 
vowels, and intelligibility is severely impaired. 

Center clipping produces subjective effects 
much more marked than those of the types of 



Figure 15. Comparison of effects of peak clip¬ 
ping and center clipping. Talker and listeners 
were located in random noise field of 110 db. 
Filled and unfilled circles distinguish two talkers. 

distortion previously described. Even slight 
amounts of center clipping give speech a 
peculiar “lisplike” quality. It has also been 
noticed that center-clipped speech sounds some¬ 
what like the speech transduced by a throat 
microphone. Increasing amounts of center clip¬ 
ping produce, first, a “coarse,” “granular” 
sound, followed by the effect of intermittent 
breaks or shorts in the circuit. Severe center 
clipping causes speech to sound rather like 
radio static. 

Inasmuch as center clipping eliminates the 
weak sounds, it is possible that use could be 
made of center-clipping circuits in training 
speakers to talk effectively in noise. A person 
listening to himself through a center-clipping 
circuit has a natural tendency to emphasize the 
weak consonants which, otherwise, he would 
not hear on his side-tone circuit. The circuit 
might also find application in connection with 
“speech interviews” in which it is desired to 


rate individuals as to their probable effective¬ 
ness in talking against severe interference. 

Linear Rectification 

The relatively small amounts of “linear rec¬ 
tification” which might occur in a poorly bal¬ 
anced Class B amplifier were found to cause 
no serious impairment of intelligibility. Articu¬ 
lation scores, as shown in Figure 16, were down 
10 to 20 per cent, however, when the speech 


PERCENT HARMONIC DlSTORTION, HO 



Figure 16. Articulation as a function of “linear 
rectification.” 


wave was half-wave rectified. Full-wave recti¬ 
fication was extremely detrimental to speech 
quality and reduced intelligibility severely. 

Applications of Peak Clipping 

It has been shown that peak clipping does not 
seriously impair the intelligibility of speech. 
In some cases, where the peak amplitude of the 
wave is maintained at a constant level, irre¬ 
spective of clipping, it is found that peak 
clipping may actually improve the communica¬ 
tion situation. At least three such cases have 
presented themselves, and research has been 
done to determine the advisability of deliberate 
peak clipping. 

One application occurs in the use of hearing 















96 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


aids. When the speech is amplified to the point 
at which it can be heard by people with severe 
deafness, the peaks of the wave may reach the 
feeling threshold of the ear. The possibility 
of using peak clipping in hearing aids to protect 
the ear at high intensity levels is considered in 
detail in Chapter 15. 

A second use for peak clipping occurs in 
amplitude modulation [AM] radio transmis¬ 
sion. In this case the higher speech peaks may 
overmodulate the transmitter, produce inter¬ 
channel interference, and overwork the modu¬ 
lator unit. Peak clipping makes it possible to 
maintain 100 per cent modulation at all times. 

The third case also occurs in the realm of 
radio telephony. As shown in Figure 14, peak 
clipping of the proper amount may improve 
the intelligibility of speech heard in interfering 
static. This application is presented in Chapter 
13, where the use of noise limiters in radio 
receivers is discussed. 

The general conclusion must be, therefore, 
that amplitude distortion is not always unde¬ 
sirable. If the interest lies in intelligibility 
rather than high quality, certain types of 
amplitude distortion may actually be advisable. 
Peak clipping, which provides greater intelligi¬ 
bility per unit peak amplitude, seems to offer 
a number of possibilities for improving com¬ 
munication efficiency. 


7 2 FREQUENCY DISTORTION 

Frequency distortion occurs whenever a 
transmission system responds unequally to 
sinusoidal signals of different frequencies. It 
is obvious, of course, that lack of uniform 
response for frequencies above or below the 
audible range cannot be detected by the ear, 
and consequently the term “frequency distor¬ 
tion” as used herein refers to the response of 
the system for audio frequencies. 


7-21 Concept of Ortliotelephonic Gain 

Meaningful comparisons between two sys¬ 
tems can be most readily obtained by defining a 
suitable reference system. A reference system 


which uses real talkers and listeners and which 
is not too difficult to reproduce in the labora¬ 
tory has been defined at the Bell Telephone 
Laboratories. 4 It is called orthotelephonic refer¬ 
ence system, and it consists of a “normal” 
talker and listener facing each other in open 
air at a distance 1 m apart in a quiet, non- 
reverberant room. 

The overall frequency response of a system 
under test is then expressed relative to that of 
the orthotelephonic system, and the response 
so obtained is called the orthotelephonic gain 
of the system. The basic notion is that the 
orthotelephonic gain of a system at a given 
frequency shall be expressed as the ratio of 
the level produced at the listener’s ear by the 
system under test to that at the listener’s ear 
when speech is transmitted from talker to 
listener over a 1-m path in free air. By defini¬ 
tion, then, speech will sound the same over a 
system having an orthotelephonic gain of 0 
db at all frequencies as it will when heard 
over an air path 1 m long. 

In practice it is often more convenient to 
obtain the overall response of the system to be 
tested in three separate steps (microphone, 
earphone, and amplifier). More often than not, 
human talkers and listeners are replaced by 
artificial voices and ears. Although approxi¬ 
mate only, such response characteristics are 
useful. For a more complete discussion, see 
Chapters 9 and 10. 


Types of Frequency Distortion 

As a matter of convenience, circuits with 
nonuniform frequency responses can be classi¬ 
fied into four types: 

1. Circuits which pass high frequencies more 
efficiently than low frequencies. 

2. Circuits which pass low frequencies more 
efficiently than high frequencies. 

3. Circuits which pass intermediate frequen¬ 
cies more efficiently than high and low fre¬ 
quencies. This is the most common type of 
frequency distortion. 

4. Circuits which pass high and low frequen¬ 
cies more efficiently than the intermediate fre¬ 
quencies. 



FREQUENCY DISTORTION 


97 


These various types of frequency distortion 
are conveniently produced for study by high- 
pass, low-pass, band-pass, and band-elimination 
filters. 

In most cases an adequate statement of the 
type of distortion can be made in terms of cutoff 
frequencies and/or the slope of the response 
characteristic over the frequency ranges of 
nonuniform amplification. These two specifica¬ 
tions are adequate, however, only in idealized 
cases. When the single-frequency response 
characteristic is complex and irregular, the only 



-60 -50 -40 -30 -20 -10 0 10 20 30 40 


ORTHOTELEPHONIC GAIN 

Figure 17. Functions relating syllable articulation 
of filters as parameter. 

adequate definition of the frequency distortion 
is the graphical representation of the charac¬ 
teristic. 


7-2 3 The Effects of High-Pass and Low-Pass 
Systems 

The effects of high-pass and low-pass circuits 
on the intelligibility of speech have been studied 
with two kinds of filtering. Data gathered sev¬ 
eral years ago at the Bell Telephone Labora¬ 
tories related the articulation score to the 
cutoff frequency of sharply discriminative 
filters. The results of a few tests of this type 
are contained in reference 2. These data are 


now supplemented by results obtained with 
circuits having a continuous slope throughout 
the range of speech frequencies. The articula¬ 
tion score can, in this case, be related to the 
slope of the “tilted” response characteristic. 

Effects of High-Pass and 
Low-Pass Filters 

The relation between articulation scores and 
frequency distortion can be investigated with 
highly discriminative high-pass and low-pass 
filters inserted in a good quality broad-band 



to orthotelephonic gain with the cutoff frequency 

transmission system. Comprehensive tests of 
this type made by the Bell Telephone Labora¬ 
tories® are summarized in Figure 17 for high- 
pass and low-pass filters. These smoothed-gain 
functions are drawn from composite results 
for both men’s and women’s voices. Nonsense 
syllables were used as test material. 

The articulation scores rise rapidly as the 
orthotelephonic gain of the passed frequencies 
increases. The optimal scores are obtained for 
each of the filter conditions at about the same 
point, roughly + 10-db orthotelephonic gain 


a These data are as yet unpublished in complete form. 
They were made available to the Psycho-Acoustic and 
the Electro-Acoustic Laboratories in order to aid the 
war effort. 


































































98 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


for the particular speaking levels used. If these 
optimal scores are now plotted against the 
filter cutoffs, the curves shown in Figure 18 
are obtained. 



FREQUENCY IN CYCLES PER SECOND 

Figure 18. Functions relating syllable articula¬ 
tion to the cutoff frequency of high-pass and low- 
pass filters. Data taken from Figure 17 at +10 
db orthotelephonic gain. 

It will be seen that the low frequencies of 
speech contribute very little to the articulation 
score in spite of the fact that they carry most 
of the speech power (see Chapter 4). For ex¬ 
ample, when all the frequency components of 
speech below 1,000 c were attenuated by a high- 
pass filter, the speech power was reduced ap¬ 
proximately 80 per cent, but the articulation 
score fell only 10 per cent. Also, it should be 
noted that, according to these data, the fre¬ 
quencies above 1,900 c make the same contribu¬ 
tion to the articulation score as do the 
frequencies below 1,900 c. 

For military purposes it is especially desir¬ 
able to know the effects of high- and low- 
frequency cutoffs when the equipment is used 
in noisy environments. Tests conducted at the 
Psycho-Acoustic Laboratory related the articu¬ 
lation score to the low-frequency cutoff, 5 and 
also to the high-frequency cutoff 5a of several 
standard military equipments. Results obtained 
for different low-frequency cutoffs with four 
microphones under a variety of noise condi¬ 
tions are shown in Figure 19. In all cases the 
effect of raising the low-frequency cutoff is to 


lower the intelligibility of speech. The loss is 
greatest when the noise conditions are least 
favorable, and in general the rule holds that 
the more difficult the conditions the wider 
should be the band of frequencies passed by the 
interphone. This result is surprising in view 
of the fact that the noise used in the tests 
simulated the spectrum of airplane noise and 
contained a large proportion of the energy in 



Figure 19. The effect on articulation of varying 
the low-frequency response. Parameters represent 
different microphones and noise conditions. 


frequencies below 250 c. Apparently, for linear 
systems it is more important to pass the lower 
speech frequencies than to exclude the lower 
components of the noise. 

Tests were also made to compare a high- 
frequency cutoff of 3,000 c with one of 4,000 c. 
Three different microphones and five different 
noise conditions were used in the tests, and 
under all conditions the cutoff at 4,000 c gave 
higher articulation scores than the cutoff at 
3,000 c. The advantage ranged from 1 to 7 per 
cent. In view of this consistent advantage, it 
was recommended that the earphone be de¬ 
signed to have a flat response as a function of 
frequency and a cutoff at a frequency not lower 
than 4,000 c. This recommendation was re- 























































































































FREQUENCY DISTORTION 


99 


sponsible for the design of the ANB-H-1 ear¬ 
phone. 

Effects of “Tilted” 

Response Characteristics 


is indicated both by the peak sound pressure 
and the average sound-pressure levels [SPL] 
(measured by a VU meter) necessary to obtain 
50 per cent word articulation in quiet. The 


A second type of frequency distortion has 
been studied at the Psycho-Acoustic Labora¬ 
tory. Filters having gradual, sloping cutoffs 
were used, and the response over the range of 
speech frequencies was “tilted” as shown in 














A 


—- J 















1 










— 

//>■ 







S 

\ 







\ 









*\ H 

5 

.''A' 








—; 












V 

AT 

T 











'—A 



^ 1 

y, 









V 

\ 

\ 


A 

\i 











\S 

\ N 

P 6 

fV 

\ 

7 










\ , 

M2- 

N J \ 

£ 

i 











^ L 

\ 


V 












k N 

'J 



FREQUENCY IN CYCLES PER SECOND 

Figure 20. Overall frequency-response char¬ 
acteristics for different tilts measured with a 6- 
cu cm coupler. 


Figure 20. The solid lines describe the overall 
response b of t]?e system when no filters were 
used and is considered “flat” or uniform. The 
broken lines indicate the response when filters 
were used to obtain high-pass and low-pass 
tilts of 6 and 12 db per octave. The relative 
levels of the tilted response characteristics are 
adjusted in such a way that all five systems 
yield the same articulation score (50 per cent 
word articulation in quiet). 

It will be noted that for equal articulation 
scores the gain of the five systems is approxi¬ 
mately the same between 1,000 and 1,500 c. 
Most of the speech power is carried by fre¬ 
quencies below 1,000 c, however, with the result 
that the intensity of the speech received via 
the low-pass systems is necessarily greater 
(for 50 per cent word articulation) than the 
intensity of the speech received via the high- 
pass systems. This fact is revealed by Figure 
21, where the intensity of the received speech 

b For purposes of calibration the acoustic output of 
the headphones was measured with a 6-cu cm coupler 
(artificial ear). 



Figure 21. Showing the peak and average 
sound-pressure levels of the received speech 
necessary in quiet to obtain 50 per cent word 
articulation with different tilts. 

received intensity of the low-pass speech is 
almost 20 db greater for the same articulation 
score. 

It can also be seen in Figure 21 that the 
difference between the peak and average levels 
of the speech (the peak factor) is smallest for 
the low-pass system, largest for the high-pass. 
The change in peak factor indicates a change 
in wave form, and this alteration of the speech 
wave is illustrated in Figure 22 for high-pass 
( + ) and low-pass ( —) systems. These oscillo¬ 
grams of the word “show” also reveal a system¬ 
atic change in the ratio of the consonant and 
vowel intensities. With low-pass tilts the 
consonant-vowel ratio is very low, but with 
high-pass tilts the consonants are relatively 
much stronger. This tends to make the speech 
wave more nearly uniform in level. 

When similar articulation data are obtained 
under noise conditions, however, the results 
are somewhat modified. In Figure 23 the 
speech-to-noise ratio (peak voltage of speech 
to rms voltage of noise) at the threshold of 
intelligibility is plotted against the tilt of the 
response characteristic. Comparison of Figures 








































100 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


21 and 23 shows that systems incorporating 
low-pass tilts provide relatively higher intelli¬ 
gibility in noise than in quiet. 

This difference is probably attributable to 
the difference between the masked and the 
quiet thresholds of hearing (see Section 3.4.1). 
In quiet the threshold of hearing is higher for 


low frequencies, and low-pass systems must 
therefore provide a higher signal level than 
high-pass systems. In intense random noise, 
however, the masked threshold is higher for 
high frequencies (see Figure 9 in Chapter 3), 
with the result that low-pass systems are more 
efficient. Such assumptions depend upon the 



-12 






.1 . , ..A. 


TIME IN SECONDS 


Figure 22. Oscillograms of the word “show” after different amounts of tilting. 












FREQUENCY DISTORTION 


101 


detectability of tones in quiet and in noise, 
however, and these do not tell the complete 
story. The three curves of Figure 23 represent 
the thresholds of intelligibility, perceptibility, 
and detectability (see Chapter 5) for continu- 



TILT IN DB PER OCTAVE 

Figure 23. Effects of tilting upon the thresholds 
of intelligibility, perceptibility, and detectability 
of continuous speech in the presence of random 
noise. 

ous speech. Extreme low-pass systems make 
speech easier to detect in the presence of noise 
but not easier to understand. 


7 . 2.4 Effects of Band-Pass Systems 

The perception of speech does not utilize all 
of the frequencies which the ear can detect, 
and for most speech communication devices it 
is economical to tailor the frequency response 
of the system to the necessary speech frequen¬ 
cies rather than to the entire audible range. 
Engineering considerations often make it 
desirable to limit the range of frequencies even 
further. It becomes necessary to know, there¬ 
fore, what is lost in intelligibility as the band 
of frequencies is made narrower and narrower, 
and it is often useful to have at hand the 
quantitative relations among intelligibility, 
bandwidth, and level of received speech, evalu¬ 
ated under the acoustic stress of interfering 
noise. 

Effects of Band-Pass Filters 

Some of these relations have been determined 
by articulation tests conducted in noise with 


communication systems having a variety of 
band-pass characteristics . 7 The test material 
(nonsense syllables) was spoken into 12 sys¬ 
tems having bandwidths ranging from about 
one-half octave to the entire range of speech 
frequencies. The responses of these systems 
were approximately uniform over the pass band 
and had sharp cutoffs at either end. The acoustic 
gain of the pass bands was expressed in terms 
of the orthotelephonic reference system. Each 
system was tested over a wide range of speech- 
to-noise ratios. In all cases the speech-to-noise 
ratio was varied by altering the gain of the 
system under test, while the voice level of the 
talker and the level of the noise in the listener’s 
ears were held constant. 

Two spectra of masking noise were used. The 
noises were intended to bracket the wide range 
of noise conditions likely to be encountered in 
military situations. The noise was introduced 
electrically into the system in a manner de¬ 
signed to simulate both the condition in which 
only the talker is in an ambient noise and the 
condition in which only the listener is in the 
noise. 

The results of the syllable articulation tests 
can be presented in the form of gain functions 
which express the relation between articulation 
efficiency and the level of the received speech. 
Perhaps a more useful form, however, is the 
equal-articulation contour which shows the 
relation between level of received speech and 
bandwidth necessary to maintain a given 
articulation score. 

Illustrative data are presented in Figures 24 
through 28. These data were obtained with 
a noise spectrum containing primarily low- 
frequency components, and represent the case 
where the listener is located in a typical 
ambient noise field (see Chapter 2). The family 
of gain functions obtained for the group of 
bands having a center 0 frequency of approxi¬ 
mately 1,100 c is shown in Figure 24. Similar 
families of gain functions were obtained for 
band-pass systems having center frequencies of 
1,500 and 2,000 c. 

If a horizontal line is now drawn through 
such a family of gain functions, the intersec- 

c The center frequency is defined here as the geo¬ 
metric mean of the two cutoff frequencies. 







102 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


tions mark off on the abscissa the values of 
gain required for each of the bands to give the 
same articulation score. Repeating this process 
for various articulation levels produces a family 



-10 0 10 20 


Figure 24. Gain functions for band-pass sys¬ 
tems in the presence of noise. 

of equal-articulation contours. In Figures 25 
to 28 are shown four sets of equal-articulation 
contours drawn from the gain functions of 
Figure 24 (supplemented by tests with several 
additional band-pass systems). 

These contours are typical of the results 
obtained with other types of noise and with 



Figure 25. Equal-articulation contours for 
band-pass systems with fixed lower cutoff at 550 
c. (Unfiltered spectrum B.) 

other center frequencies. Since in each case the 
family of contours applies only to the specific 
test conditions, an attempt was made to derive 
from the data a set of generalized equal-articu¬ 
lation contours applicable to a wider variety of 


cases. The derivation of these functions involves 
a certain amount of smoothing and averaging, 
but in spite of this it is felt that the generalized 
functions serve a useful purpose in illustrating 
the general nature of the relations between 
bandwidth level and articulation. 

The generalized equal-articulation contours 
were all plotted relative to the level of the wide¬ 
band system (130 to 9,200 c) required to give 
an equivalent articulation score. The value of 
0 -db relative gain was thus assigned arbi¬ 
trarily to all points of the gain function for the 
wide band. By this procedure families of con¬ 
tours were constructed for bands with center 
frequencies at 1,100, 1,500, and 2,000 c for 
each noise condition tested. Since the resulting 
sets of contours showed a remarkable simi¬ 
larity, it was possible to represent all of them 
by the single generalized family of contours 
shown in Figure 29. This generalized family of 
contours was constructed with a center fre¬ 
quency of 1,500 c, but to a reasonable approxi¬ 
mation the same contours can be used for other 
center frequencies by shifting the entire family 
horizontally one w 7 ay or the other on the semilog 
plot. The contours probably should not be used 
for bands having center frequencies outside 
the range from about 1,000 to 2,000 c. 



Figure 26. Equal-articulation contours for 
band-pass systems with fixed lower cutoff at 870 
c. (Unfiltered spectrum B.) 


The use of these contours may be illustrated 
by example. Suppose that a high-fidelity com¬ 
munication system is to be replaced by a system 
passing only the frequencies from 750 to 3,000 
c. Suppose further that these systems are used 














































































































































































FREQUENCY DISTORTION 


103 


in a noise having a flat or falling spectrum 
similar to the types tested and that it is de¬ 
sired to determine the increase in gain required 
to maintain the articulation at 50 per cent. Then 
entering Figure 29 at the abscissas of 750 and 
3,000 c shows that a gain of 15 db is required 
to maintain articulation at 50 per cent. d If, 
however, an increase of only 10 db is prac- 



Figure 27. Equal-articulation contours for 
band-pass systems with fixed upper cutoff at 
2,500 c. (Unfiltered spectrum B.) 

ticable, a band from 640 to 3,500 c is required 
to maintain 50 per cent articulation. 

It should be emphasized that these contours 
are intended to furnish only approximate re¬ 
sults. They were obtained with idealized pass 
bands and should be generalized to other types 
of band-pass systems with some caution. They 
apply to those instances in which the spectrum 
of the masking noise extends fairly uniformly 
over the major part of the audio-frequency 
range. Since there is evidence that articulation 
is determined more by the speech-to-noise ratio 
than by the absolute level of the noise, the con¬ 
tours are not restricted to the particular noise 
and speech levels used in obtaining the data. 
By the use of proper transformations, 3 these 
results obtained with nonsense syllables will 
be valid for other types of test materials. 

It may be stated as a general conclusion that 
an adequate speech-to-noise ratio should be 
provided over as wide a frequency range as 


d If the center frequency is, for example, 1,200 c 
instead of 1,500 c, multiply the two cutoff frequencies 
by the ratio of 1,500 to 1,200 and enter Figure 29 with 
the resulting frequencies. 


possible. For ideal speech transmission the 
frequency range should extend from about 200 
to 7,000 c, and the signal-to-noise ratio at each 
frequency should be 25 db or more. 

Effects of Resonant Peaks 

A special case of band-pass frequency dis¬ 
tortion is provided by resonant earphones. The 



Figure 28. Equal-articulation contours for 
band-pass systems with fixed upper cutoff at 
3,900 c. (Unfiltered spectrum B.) 


typical earphone resonance occurs between 
1,000 and 2,000 c and can, therefore, be evalu¬ 
ated in terms of the generalized equal-articula- 



Figure 29. Generalized equal-articulation con¬ 
tours plotted for band-pass systems all having 
a center frequency of 1,500 c. 


tion contours. The effects of resonant peaks 
above or below this frequency range have been 
studied in less detail, and results indicate that 
a low resonant frequency is more damaging to 
intelligibility than a high-frequency resonance. 






































































































































































































104 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


Illustrative data were obtained with the four 
response characteristics shown in Figure 30 
(response measured with a 6-cu cm coupler). 
These characteristics provide a basis for com- 



Figure 30. Single frequency-response character¬ 
istics for flat and resonant systems. 


paring a relatively flat or nonresonant system 
with systems resonant at low (375 c), inter¬ 
mediate (1,050 c), and high (2,500 c) fre¬ 
quencies. Word-articulation results, using the 
check-list method with 90 db SPL of random 



Figure 31. Articulation as a function of signal- 
to-noise ratio for resonant systems shown in 
Figure 30. 


noise, are shown in Figure 31. None of the 
resonant systems is as effective as the non¬ 
resonant system. 

The most important objection to resonant 
systems, however, arises when communication 
is attempted in intense ambient noise. If the 
level of the speech through the system is high 


enough to override the noise, the level of the 
received speech at the resonant peak is painful 
to the listener. As a general rule, therefore, it 
is advisable to determine that the overall re¬ 
sponse of a communication system is free of 
prominent resonant peaks. 


-3 COMBINED FREQUENCY AND 
AMPLITUDE DISTORTION 

In practice both frequency and amplitude 
distortion may occur in the same system, and 
it is necessary to know the importance of their 
interactions. The possible permutations of fre¬ 
quency distortions, amplitude distortions, and 
noise conditions are so numerous, however, that 
this line of investigation has really only begun. 
Consequently, this section can be only sug¬ 
gestive rather than definitive. 


Effects of Tilting and Clipping 

Frequency distortion of the type referred 
to in Section 7.2.3 as “tilting” was tested with 
various amounts of sharp, symmetrical peak 
clipping. The response characteristic of the 
entire system in the absence of nonlinear 
distortion is shown in Figure 19, and these 
can be conveniently referred to as HP-12 (high- 
pass system, rising 12 db per octave), HP-6, 
flat, LP-6 (low-pass system, falling 6 db per 
octave), and LP-12. The speech signal was first 
passed through the tilting network, then peak- 
clipped and fed to the listener’s earphones. The 
clipping level was held constant at 112 db 
SPL, and no speech peaks above this level 
reached the listener’s ears. 

The results of word articulation tests made 
in quiet conditions over these systems are 
shown in Figure 32. Speech intensity is indi¬ 
cated in terms of the number of decibels the 
peak amplitude of the signal (as read on an 
oscilloscope) fell below the fixed clipping level. 
As the speech intensity is increased, the per 
cent word articulation rises to a maximum 
which is maintained up to the level at which 
clipping begins. Below the clipping point there 
is little difference between the tilts, although 











































COMBINED FREQUENCY AND AMPLITUDE DISTORTION 


105 


LP-12 appears somewhat inferior. Above the 
clipping point, however, large differences ap¬ 
pear in the susceptibility of the different tilts 
to peak clipping. The LP-12 and LP-6 tilts show 
a marked deterioration in intelligibility as the 


noise were then tilted and clipped; the amount 
of peak clipping was measured from the point 
at which the combined signal was limited. The 
results of these tests are shown in Figure 33 
for the HP-6, flat, and LP-6 tilts. In the 



Figure 32. Word articulation in quiet as a function of speech intensity with frequency response as the 
parameter. Zero db (112 db SPL) represents the level at which clipping began. 


amount of peak clipping is increased. The flat 
and high-pass systems, on the other hand, 
remain intelligible with as much as 60-db clip¬ 
ping. Apparently a response characteristic 
which rises at about 6 db per octave gives the 
smallest loss in intelligibility with severe peak 
clipping. 

Similar tests were run under noise condi¬ 
tions, using a dense, crackling static. The noise 


absence of nonlinear distortion the flat response 
is superior, but when clipping is introduced the 
HP-6 tilt gives slightly higher articulation 
scores. 

The choice of 112 db as the clipping level in 
these tests is arbitrary and does not greatly 
affect the experimental results. Essentially 
similar functions were obtained with higher 
clipping levels. 



Figure 33. Word articulation in noise as a function of speech intensity with frequency response as the 
parameter. The signal-to-noise ratio at the microphone was +10 db. 

along with the speech was picked up by the 7 - 3 - 2 Application to Speech at High 
microphone, and the talker monitored his Altitudes 

speech level by a VU meter in such a way that 

a signal-to-noise ratio of + 10 db was main- Two special problems arise when communica- 
tained at the microphone. Both speech and tion is attempted at high altitudes (see Chapter 






























106 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


9 and Section 4.1.3). The overall speech level 
drops considerably, and the quality of the 
speech deteriorates. Changes in the overall level 
as a function of altitude necessitate either some 
sort of altitude-controlled amplification or a 
compression amplifier to reduce the range of 
differences. Also quality changes due to reduced 
ambient pressure and to the oxygen-mask 
enclosure suggest that some low-frequency 
attenuation might be desirable, especially when 
carbon microphones are used. In order to ex¬ 
plore these possibilities, tests were run using 
a variety of conditions of frequency and ampli¬ 
tude distortion. 

Several voices were phonographically re¬ 
corded for test. Talkers with A-14 oxygen 
masks were located in an altitude chamber, and 
speech samples were obtained at 5,000 ft (sea 
level) and at 35,000 ft, using a carbon micro¬ 
phone (ANB-M-C1). Frequency distortion was 
introduced in the playback amplifiers. The 28 
different response characteristics tested are 



FREQUENCY IN CYCLES PER SECOND 

Figure 34. Single frequency-response character¬ 
istics used in testing the intelligibility of speech 
at high altitudes. 


indicated in Figure 34. The low-frequency cut¬ 
off varied from 500 to 4,000 c, and the slope of 
the cutoff was 6, 12, or 17 db per octave. The 
two high-frequency cutoffs were 3,200 and 
5,000 c. Random noise interference was intro¬ 
duced electrically after the speech signal had 
been filtered, and the intensity of the noise was 


held constant at a level corresponding to 110 
db re 0.0002 dyne/cm 2 . The peak speech inten¬ 
sity was determined with a calibrated cathode- 
ray oscilloscope. The signal-to-noise ratio is 
expressed as the difference in decibels between 
the peaks of the speech and the rms level of 
the noise. 

Results obtained in the absence of peak 



Figure 35. Showing threshold of intelligibility 
as a function of the high-pass cutoff frequency, 
with slope of cutoff as parameter. Low-pass cut¬ 
off was 5,000 c. 


clipping are shown in Figure 35 where the 
signal-to-noise ratio at the threshold of intelli¬ 
gibility is plotted in decibels against the low- 
frequency cutoff with the slope of the cutoff 
as the parameter. The two high-frequency cut¬ 
offs tested gave similar results. 

Accentuation of the high-frequency compo¬ 
nents of the speech does not lower the signal-to- 
noise ratio at the threshold of intelligibility 
(see Section 7.2.3). The quality seems better, 
however, as the “booming” caused by the mask 
enclosure is reduced. As can be seen in Figure 
35, the effect of the high-pass filter is most 
pronounced for the steeper cutoff slopes. 

However, little or no improvement in the 
intelligibility of altitude speech is obtained by 
deliberate frequency distortion. It is probable 
that the altitude, the mask enclosure, and the 
extraneous valve noises have more extensive 
effects upon speech sounds than is indicated 
by frequency-response characteristics. Conse¬ 
quently, simple equalization of the response 
characteristic does not produce a dramatic 
improvement in performance. 

Tests were next conducted with the addition 
of peak clipping to the frequency distortion. 
The altitude speech was first filtered to produce 
the desired frequency distortion, and then the 

































































































COMBINED FREQUENCY AND AMPLITUDE DISTORTION 


107 


speech peaks were clipped 0, 10, 20, or 30 db. 
Random noise was mixed electrically with the 
peak-clipped speech, the intensity of the noise 
being constant at a level corresponding to 90 
db re 0.0002 dyne/cm 2 . The check-list method 
of articulation testing was used. 

In Figure 36 the articulation obtained with 



< 


O 



frequencies. If this signal is now peak clipped, 
the intelligibility of the speech declines in a 
manner similar to the decline shown in Figure 
32 for the low-pass conditions. When the low 
frequencies are attenuated by a 500-c high-pass 
filter, however, peak clipping does not sig¬ 
nificantly change the intelligibility of the 




RATIO IN DECIBELS OF SPEECH PEAKS TO RMS NOISE LEVEL 

Figure 36. Word articulation as a function of peak-speech-to-rms-noise ratio with altitude, frequency 
response, and peak clipping as parameters. A carbon microphone (ANB-M-C1) was used in the oxygen 
mask (A-14). 


a low-frequency cutoff at 80 c is compared 
with results obtained using a 500-c cutoff. In 
the absence of peak clipping, the attenuation 
of frequencies below 500 c does not seriously 
affect articulation scores either at altitude or 
sea level (see Figure 35). This is also true 
for sea-level speech when clipping is intro¬ 
duced. When the high-altitude speech is peak 
clipped, however, the effect depends in large 
part upon the frequency response of the system. 
When a flat system is used, the mask and car¬ 
bon microphone combine to accentuate the low 


altitude speech relative to the intelligibility of 
sea-level speech. 

To apply these data to the practical problems 
of communication in and between airplanes, 
account must be taken of the fact that the signal 
intensity at altitude is about 10 db below the 
intensity at sea level. If a clipping amplifier 
were designed, for example, to clip sea-level 
speech 30 db, the speech at altitude would be 
clipped only about 20 db. A comparison of the 
appropriate functions in Figure 36 (20-db 
clipping at altitude vs 30-db clipping at 






























108 


INTELLIGIBILITY OF SPEECH: EFFECTS OF DISTORTION 


sea level) shows that about 8 db more amplifi¬ 
cation is needed at altitude to reach 50 per 
cent articulation S/N = + 1 db) than is needed 
at sea level ( S/N = — 7 db). This compares 
with 15 to 20 db of additional amplification 
required at altitude to give equivalent articu¬ 
lation scores when peak clipping is not used. 
Thus the range of signal levels which the inter¬ 
phone or modulator is required to handle is 
substantially reduced. 

Peak clipping has a further advantage in the 
case of AM radio-telephony at high altitudes, 
because the clipping level can be adjusted to 
give 100 per cent modulation of the carrier at 
all times. The disadvantage arises from the 
very severe clipping of sea-level speech which 


is required in order to obtain adequate limiting 
of the weaker speech signal at altitude. Com¬ 
promises are obviously necessary. 

Results reported in this chapter have indi¬ 
cated that it is often possible to take liberties 
with speech as the talker produces it, and that 
distortion, particularly nonlinear distortion, is 
not always undesirable. The question which 
arises for the concern of future investigators 
is, therefore, what changes can be made in a 
typical speech signal to put it into optimal form 
for any given communication situation. Can 
speech be packaged to fit more neatly into avail¬ 
able communication equipments? The problem 
of finding the best speech package for every 
situation is an extensive undertaking. 



Chapter 8 

INTELLIGIBILITY OF SPEECH: TYPES OF INTERFERENCE 


M ilitary communications are normally 
subject to a wide variety of interfering 
noises. A ship-to-plane radio-telephone link, 
for example, may be simultaneously afflicted 
by pulse interference from shipborne radar 
and other electrical equipments, atmospheric 
static, tube noise in the radio receiver, and 
the roar of the plane’s engines or the ship’s 
guns. It is reasonable to suppose that some of 
these noises may be tolerable while others pro¬ 
vide a serious cause for concern. It is neces¬ 
sary, therefore, to explore an extensive range 
of signals to determine what factors contribute 
to their effectiveness for masking speech. 


81 GENERAL CONSIDERATIONS 

As far as the ear is concerned, the important 
dimensions of an interfering noise are its 
intensity, spectrum, temporal continuity, and 
annoyance value. The most important of these 
is intensity, for if the noise is 20 or 30 db less 
intense than the speech signal, other aspects 
of the interference become irrelevant. With a 
given interference of sufficient intensity, how¬ 
ever, the amount of masking depends upon the 
spectrum and the temporal continuity of the 
noise. 

Masking is customarily expressed in terms 
of the shift in the threshold of hearing which 
results when an interfering signal is intro¬ 
duced (see Section 3.4). However, the signal- 
to-noise ratio can also be used as a measure 
of the relative effectiveness of different types 
of interference. In this sense the masking effi¬ 
ciency of a signal is defined in terms of the 
percentage of the words which cannot be heard 
at a given signal-to-noise ratio. Thus it is 
possible to state, for example, that noise A 
masks the same percentage of words at a signal- 
to-noise ratio of 2 db as noise B masks at a 
signal-to-noise ratio of 10 db, i.e., noise A is 
8 db more effective than noise B in masking 
speech. 


811 Spectrum of the Interfering Noise 

The effective component frequencies of 
speech are those ranging between 200 and 7,000 
c and, in general, a signal whose spectrum is 
correspondingly wide has the maximum mask¬ 
ing efficiency. Before considering these complex 
sounds, however, it is useful to review some 
of the information available on the masking of 
pure tones by pure tones. 

The simplest case of masking is encountered 
when two pure tones are introduced into the 
ear and the observer is instructed to adjust the 
intensity of one of these tones until it is just 
detectable. 1 It is found that the presence of 
a second tone tends to reduce the ability of the 
ear to hear the first tone and it is, therefore, 
necessary for the intensity of the first tone to 
be increased before the listener can detect it. 
How much the intensity must be increased 
will depend upon the intensity of the second 
or masking tone and also upon the frequency 
relation between the two tones. The experi¬ 
mental results lead to two general conclusions. 
(1) The masking produced by a tone is greater 
for frequencies above the frequency of the 
masking tone. (2) The spread of the masking 
to the higher frequencies is most pronounced 
at high intensities of the masking tone. These 
effects are due, at least in part, to the nonlinear 
operation of the ear at high intensities. 

In general, the masking effectiveness of a 
narrow band of noise is also greatest for 
frequencies in or above the band, and the 
masking increases rapidly at high intensities. 
When the masking signal is a band of noise, 
however, both the center frequency and the 
width of the frequency band determine the 
masking effectiveness. If the masking signal 
covers a narrower range of frequencies than 
the masked signal, the frequencies of the 
masked signal which lie outside the range of 
the masking signal may be heard. If the spec¬ 
trum of the masking signal is much wider than 
that of the signal to be masked, the outlying 


109 


no 


INTELLIGIBILITY OF SPEECH: TYPES OF INTERFERENCE 


frequency components of the masking signal 
do not contribute to the masking. Logically, 
there must be some optimal spectrum for the 
most efficient masking of a given signal, and 
this spectrum will depend upon the range of 
frequencies in the masked signal. 

The problem is, therefore, to apply these 
general principles derived from pure-tone 
masking to the masking of the speech spec¬ 
trum. If pure tones are used to mask speech, 
it is obvious that tones of low frequency will 
be more effective than tones of high frequency, 
because at high intensities the masking effects 
of a low-frequency tone will extend upward in 
frequency to cover the entire range of speech. 
From the general principle that the optimum 
frequency range of the masking signal is a 
function of the frequency range of the signal 
to be masked, it is apparent that even the most 
effective single frequency will not efficiently 
mask the wide frequency range of speech. If, 
therefore, bands of noise are used to mask the 
speech, the masking efficiency will be improved. 
The frequency relations between the masking 
and the speech signals are such that the maxi¬ 
mum efficiency is produced by masking signals 
which cover the frequency range of speech and 
which have the maximum interfering intensity 
concentrated at the low end of this range. 

It should be noted that, within certain broad 
limits, the masking efficiency of a noise is 
independent of the phase relations among the 
various component frequencies making up the 
spectrum. The actual form of the masking wave 
as seen on an oscilloscope is of little consequence 
in determining the masking efficiency. This 
agrees with the general notion that the ear is 
relatively insensitive to phase relations. 


Temporal Continuity of the 
Interfering Noise 

The nature of speech and the operation of 
the ear are such that relatively large portions 
of the speech can be completely “blanked out” 
in time without seriously lowering the intelligi¬ 
bility of the speech. 

This generalization may be experimentally 
demonstrated in either of two ways. The speech 


signal can be turned on and off intermittently, 
and the loss in intelligibility can then be ex- 
expressed as a function of the percentage of 
time the signal was available to the ear. Or an 
interfering noise may be intermittently inter¬ 
rupted and the loss in intelligibility can be 
expressed as a function of the percentage of 
the total time during which the speech signal 
is not masked by the noise. 

When an electronic switch was used to inter¬ 
rupt speech at a rate of nine interruptions per 
second, little loss in word articulation was 
obtained until relatively large portions of the 
speech were missing. When only 25 per cent 
of the speech pattern was present it was still 
possible to understand approximately half the 
words. This relation is shown in Figure 1. 



PERCENT OF TIME SPEECH WAS ON 


Figure 1. Showing increase in intelligibility as 
a function of the per cent of the time the speech 
wave is unattenuated. The switching rate was 
nine interruptions per second. 

In the case where the speech was continuous 
but an interfering noise was periodically inter¬ 
rupted at a rate of nine times per second, the 
masking efficiency was found to be directly 
related to the portion of the time the noise 
was on. The relation between articulation and 
the intensity of the noise is shown in Figure 
2, where the percentage of the time the noise 





TONAL INTERFERENCE 


111 


is on is taken as the parameter. A noise which 
is present only 25 per cent of the time produces 
practically no masking of speech. 



SIGNAL-TO-NOISE RATIO IN DB 


Figure 2. Showing the masking efficiency of a 
noise signal which is interrupted for different 
proportions of the time at a rate of nine inter¬ 
ruptions per second. 


81 ‘ 3 Annoyance Value of the Interfering 
Noise 


It is obvious, of course, that the masking 
efficiency of a signal is more basic to its inter¬ 
ference with communications than is its 
annoyance value. Annoyance might force a 
listener to take off his headphones, but the 
probability of such an event cannot be predicted 
from laboratory tests. It should be noted, how¬ 
ever, that many signals, although of little 
masking effectiveness, do have characteristics 
which are unpleasant and annoying. In general 
the annoyance value of a signal increases as the 
loudness is increased, the pitch is raised, or the 
signal is changed irregularly in pitch or loud¬ 
ness (see Section 2.4). Signals having such 
characteristics will usually not prevent a de¬ 
termined listener from receiving the message, 
but from the point of view of listener efficiency 
such noises should be avoided wherever pos¬ 
sible. 


* a TONAL INTERFERENCE 

To classify the various types of signals which 
have been investigated, at least three categories 
of sounds are useful: tones, noises, and voices. 


This rather arbitrary classification of the 
various sounds serves to organize the following 
discussion of the different types of interfering 
signals. 


Pure Tones 

Simple sinusoidal tones do not effectively 
mask speech. Since such interference is some¬ 
times encountered, however, it is worth while 
to determine what frequencies have the most 
deleterious effects upon intelligibility. The re¬ 
lation between the frequency of the tone and 
the shift in the threshold of perceptibility, with 
the intensity of the tone as a parameter, is 
demonstrated in Figure 3. For these determina¬ 
tions, dynamic earphones (Permoflux PDR-10) 
in supra-aural cushions (MX-41/AR) were 
used, and the intensity of the tone was meas¬ 
ured by the voltage applied to the earphones. 
The bottom curve of this family (—50 db re 
1 v peak to peak across the earphones) corre¬ 
sponds approximately to a sound-pressure level 
of 62 db re 0.0002 dyne/cm 2 . At this level a 
pure tone has a negligible effect upon the 
threshold of intelligibility for continuous 
speech. As the intensity of the tones is in¬ 
creased above this level, however, the shift in 
the threshold of perceptibility becomes con¬ 
siderable. The occurrence of masking only at 
high intensities is understandable in terms of 
the overloading of the ear and the resultant 
nonlinear distortion. With the introduction of 
overtones, the effective frequency coverage of 
the masking signal is increased. 

It will also be noted that the maximum mask¬ 
ing effectiveness is obtained at different fre¬ 
quencies, depending upon the intensity of the 
tones. At the lowest intensities the maximum 
effectiveness is obtained with tones in the 
neighborhood of 500 to 600 c. For the more 
intense tones, however, the maximum effective¬ 
ness is obtained with frequencies between 300 
and 400 c. 

The earphone cushions permitted a small 
leak which lowered the low-frequency response 
of the earphones when coupled to the listener’s 
ear. The dotted curves have been added to 
indicate the masking which would presumably 
be obtained if a constant sound-pressure level 





112 


INTELLIGIBILITY OF SPEECH: TYPES OF INTERFERENCE 


were generated. These corrections are based 
upon threshold calibrations and can be rather 
accurately made for sine waves. In the follow¬ 
ing discussion of complex tones, however, the 
effect of cushion leaks is not taken into account. 


822 Complex Tones 

When speech is masked by complex tones, 
the role of the aural harmonics introduced at 
high levels by distortion in the ear is less sig- 



Figure 3. Showing the masking of speech by sine waves of various frequencies, with the level of the 
sine wave as the parameter. 



Figure 4. Showing the masking of speech by square waves of various frequencies, with the level of the 
square wave as the parameter. 




















































































































TONAL INTERFERENCE 


113 


nificant. With square waves or trains of pulses 
the harmonic content of the signal is sufficiently 
great that the introduction of further har¬ 
monics by the ear contributes relatively little 
to masking. 

The shift in the threshold of perceptibility 
of speech masked by square waves of different 
frequencies is shown in Figure 4, with the 
intensity of the square wave as the parameter. 
As in the case of the pure-tone masking signal, 
the high frequencies are relatively ineffective 
masking agents. At low frequencies, greater 
masking is produced by a square wave than 
by a pure tone. A square wave with the sound- 
pressure level corresponding to approximately 
115 db, measured in a 6-cu cm coupler, shifts 
the threshold of perceptibility approximately 
68 db. 

Similar results are obtained if trains of short 
pulses are used. Figure 5 shows the results of 


the frequency of the train of pulses produces 
greater separation in time between successive 
pulses but simultaneously produces less separa¬ 
tion between the component overtones. At very 
low frequencies, therefore, the frequency cover¬ 
age is excellent but intermittent, and, when 
the successive pulses are separated by too long 
an interval, speech can be heard between the 
successive pulses, and the masking efficiency 
falls rapidly. 

Figures 3, 4, and 5 are directly comparable 
in terms of the peak voltages applied to the 
earphones. In all three cases, the top curve 
(0 db) is obtained with a signal of 1 v peak 
to peak across the dynamic transducers 
(Permoflux PDR-10). For the sine and square 
waves the sound-pressure levels quoted are the 
equivalent levels generated in a 6-cu cm coupler. 
For pulses of constant amplitude, however, the 
rms pressure varies directly with the pulse 



Figure 5. Showing the masking of speech by 10-/xsec pulses at various pulse repetition frequencies, 
with the peak level of the pulse as the parameter. 


tests using a pulse 10 ^sec long. For such a 
train of pulses all the multiples of the funda¬ 
mental frequency are present in amplitudes 
equal to that of the fundamental, and, conse¬ 
quently, the pulse signal covers effectively the 
entire range of speech frequencies. Lowering 


repetition rate. When the peak is held constant, 
the rms pressure rises 3 db per octave as the 
pulse repetition frequency [PRF] is raised. 
Consequently, this correction must be made if 
the curves are to be compared on the basis of 
equivalent sound-pressure levels. So compared, 




















































114 


INTELLIGIBILITY OF SPEECH: TYPES OF INTERFERENCE 


however, the pulses have the greatest and sine 
waves the least masking effect. 


8 ' 2 ' 3 Patterns of Tones 

In addition to pure or complex tones of con¬ 
tinuous character, signals are sometimes en¬ 
countered which provide a pattern of tones of 
different frequencies. In general there are three 
types of patterns which may be encountered: 
stepped tones, warble tones, and musical sig¬ 
nals. 

Stepped-Tone Signals 

At least eight variables can be distinguished 
for the type of tonal signal which switches 
abruptly from one frequency to another. Each 
of these variables was experimentally investi¬ 
gated 3 to determine (1) its contribution to the 
annoyance produced by stepped signals, and 
(2) its role in determining the masking effi¬ 
ciency of the signal. The eight variables, along 
with their effects on the annoyance and masking 
effectiveness of the signal, are summarized in 
Table 1. 


Warble-Tone Signals 

A similar study was made of the various 
aspects of a tonal signal which changes gradu¬ 
ally—warbles—from one frequency to another. 
The results for this type of tonal pattern are 
summarized in Table 2. 

Musical Signals 

Recordings of popular dance music were 
also studied for their masking effectiveness. 
The masking of speech is not great, because the 
speech can be heard in the pauses between 
musical phrases and because clarinet and 
trumpet solos usually fall above the frequency 
range which produces the most effective mask¬ 
ing. When tests were run with two selections 
played simultaneously through a compression 
amplifier (about 10 db of compression reduces 
the range of variation in the signal level), the 
masking effectiveness of the music was ap¬ 
proximately equivalent to the masking pro¬ 
duced by the stepped and warbled signals. 

The results with these tonal patterns are 
consistent with the general considerations dis¬ 
cussed in Section 8.1. Speech is masked most 


Table 1. Summary of results for stepped-tone interference. 


Variable studied 


Effect on annoyance value 


Effect on masking efficiency 


Frequencies of the stepped 
tones 

Frequency range between 
highest and lowest 
stepped tones 

Addition of other steady 
tones 


Complexity of wave form 


Number of stepped tones 
in pattern 

Number of repetitions of 
pattern per minute 

Relative durations of each 
stepped tone 

Length of switching in¬ 
terval elapsing between 
stepped tones 


High frequencies are more annoying than 
low frequencies. 

Wide range of frequencies tends to in¬ 
crease the annoyance value. 

Decrease in annoyance, unless the steady 
tone is near enough in frequency to 
stepped tones to produce dissonance and 
beats. 

Sharp wave forms are more unpleasant 
than sinusoidal waves. 

Little effect, although many steps are 
slightly less annoying than only three or 
four steps. 

Slow rates are slightly more annoying 
than rapid rates. 

Rhythms may become more distracting 
than equal steps. Random changes in the 
switching speed increase the annoyance. 
A switching click is annoying. The pres¬ 
ence of a silent interval in the pattern in¬ 
creases the annoyance. 


Low frequencies produce more masking of 
speech than do high frequencies. 

A wide range of frequencies must involve 
some high frequencies, thus lowering the 
masking efficiency. 

Increases frequency coverage, gives 
better masking, especially if it is of low 
pitch and rich in overtones. 

Complex waves have more overtones, 
cover wider frequency range, increase 
masking efficiency. 

No effect, as long as all steps individually 
are of equal masking efficiency. 

No effect. 

No effect. 


For optimal masking, no silent interval 
should occur between steps. Masking is 
increased by reducing the switching in¬ 
terval to a minimum. 







NOISE INTERFERENCE 


115 



Table 2. Summary of results for warble-tone interference. 

Variable studied 

Effect on annoyance value 

Effect on masking efficiency 

Center frequency 

High frequencies are more annoying 
than low. 

Low frequencies mask speech more effec¬ 
tively than high. 

Frequency range 

A wide range is slightly less pleasant 
than a narrow range. 

A wide range is apt to include some in¬ 
efficient frequencies and thus may lower 
the masking effectiveness. 

Rate of warble 

Slow rate is slightly more annoying. 

Slow rate is slightly more effective. 

Manner of change 

Irregular or random warbles are most 
annoying. 

No effect as long as frequency range is 
relatively narrow. 

Addition of steady tone 

Steady tones tend to lower the annoy¬ 
ance value. 

Tones added above or below the warble 
frequencies improve the frequency cover¬ 
age and increase masking effectiveness. 


effectively when the fundamental frequency of 
the masking signal is in the range from 200 
to 500 c and when the signal is continuous in 
time and rich in harmonics. Under these condi¬ 
tions, tonal signals are only 2 or 3 db less 
effective for masking than random (“white”) 
noise and are much more annoying to the 
listener. 

8 3 NOISE INTERFERENCE 

We have seen that the maximum interference 
with speech communication is produced by a 


signal which continuously blankets the fre¬ 
quency range of speech. The signal which is 
most likely to provide this continuous coverage 
and which is a standard hazard in military 
communication is noise, i.e., interference de¬ 
void of tonal character. 

8 ' 3-1 Modulated Pulses 

A comparison between tonal and noise inter¬ 
ferences can be obtained by the use of modula¬ 
tion. If we start with a “tonal” signal consisting 
of a steady train of pulses, it is possible to 



100 IOOO 10,000 

FREQUENCY IN CYCLES PER SECONO 


Figure 6. Showing the masking of speech by modulated pulses of 10 nsec duration, with per cent interval 
modulation as the parameter. 
















































116 


INTELLIGIBILITY OF SPEECH: TYPES OF INTERFERENCE 


change the intervals between pulses by modu¬ 
lation with random noise, and to increase the 
modulation until the steady buzzing quality of 
the pulses comes to resemble the noisiness of 
very dense static (see Section 3.4.3). The 
qualitative change in the pulses is accompanied 
by a simultaneous increase in the masking 
effectiveness of the signal. The amount of in¬ 
crease in masking depends upon the frequency 
of the steady train of pulses, the increase being 
greatest for high frequencies. 

Results of threshold articulation tests run 
under these conditions are shown in Figure 6. 
The peak voltage of 10-psec pulses was held con¬ 
stant at 1.5 v, and the amount of modulation 
was varied for frequencies between 250 and 
2,000 c. The amount of modulation was ex¬ 
pressed in per cent of the modulated frequency. 
Thus, a tone of 1,000 pulses per second [pps] 
is said to be modulated 10 per cent if the short¬ 
est and longest intervals between pulses corre¬ 
spond to PRF’s of 900 and 1,100 pps. The noise 
used to modulate the pulses consisted of a band 
of frequencies from 20 c to half the pulse 
repetition rate. 

Inspection of Figure 6 shows that a very 
slight amount of modulation greatly increases 
the masking. The increased masking is presum¬ 
ably attributable to the more uniform coverage 
of the frequency range by the modulated signal. 
A little interference at all frequencies is 
apparently more damaging to speech communi¬ 
cation than a great deal of interference con¬ 
centrated at only a few frequencies. 

8 ' 3 ' 2 Random Noise 

A more representative type of noise encoun¬ 
tered in practice is random, or “white” noise. 
Considerable experimental evidence has accu¬ 
mulated concerning the effects of both ambient 
and electrical random noise, and the attempt 
has been made to determine the minimum 
speech-to-noise ratio which will allow accurate 
perception of speech under conditions similar to 
those encountered in military aviation. This 
minimum ratio depends upon the character of 
the noise, the level of the speech, and the 
amount of distortion in the equipment. Over a 
wide range of conditions, however, a speech- 


to-noise ratio of + 6 db permits the listener 
to receive connected discourse with reasonable 
accuracy. However, when listening to isolated 
words at this signal-to-noise ratio, the listener 
correctly perceives about ten words per hun- 



Figure 7. Showing the effect of the overall level 
of the white noise on the thresholds of intelli¬ 
gibility and of detectability of speech. 


dred fewer than with a more favorable ratio. 
For this reason the specification of a tolerable 
level of background noise should probably be 
set no less than 6 db below the level of the 
received speech. 2 


Table 3. Comparison of the masking of speech 
and of tones by random noise. 


Sensation 
level of 
noise 

Average shift 
in threshold of 
detectability and 
threshold of 
intelligibility 

Average shift 
in threshold 
for tones, 500, 
1,000, and 
2,000 c 

10 

1.4 


20 

5.3 

3.1 

30 

11.8 

12.4 

40 

21.0 

21.3 

50 

30.7 

31.2 

60 

40.5 

40.7 

70 

50.9 

51.0 

80 

60.4 

60.8 

90 

69.8 

69.7 


Threshold articulation tests run with a linear, 
wide-band system using random noise to mask 
speech 4 typically give the results shown in 












NOISE INTERFERENCE 


117 


Figure 7. These data represent the thresholds 
of intelligibility and of detectability for differ¬ 
ent sensation levels of the noise. Above a sen¬ 
sation level of 30 db of noise (corresponding 
to sound-pressure level of approximately 0 db 
per cycle), a 10-db increase in the noise results 
in a corresponding 10-db rise in the masking. 

Upon comparison of the masking of speech 
by noise (see Figure 7) and the masking of 
tones by noise (see Figure 10 in Chapter 3), 
it is obvious that the masking effects are simi¬ 
lar. This correspondence is particularly close 
if the average masking at 500, 1,000, and 
2,000 c is compared with the average shift of 
the thresholds of intelligibility and of detecta¬ 
bility. Table 3 shows the correspondence. 

Narrow Bands of Noise 

By the use of highly discriminative filters it 
was possible to obtain narrow bands of noise. 


each band. The results of these tests are shown 
in Figure 8, along with the frequency limits 
of the bands of noise used. No single band of 
frequencies tested was more effective in mask¬ 
ing speech than was unfiltered noise. Bands of 
noise above 1,000 c failed to mask completely 
all the speech sounds. 

These results for narrow bands of noise 
agree in general with the results obtained with 
tonal signals used to mask speech, for in both 
cases sounds of lower frequency (from 100 to 
1,000 c) are more destructive to intelligibility 
than are the higher frequencies. Considerations 
of this sort led to the prediction that the noise 
which would produce the greatest interference 
with speech communications would have energy 
at frequencies between 100 and 4,000 c, but 
with most of the energy concentrated below 
1,000 c. Actual experimentation with a variety 
of wide-band noise spectra confirmed this ex- 



Figure 8. Relating per cent word articulation to noise level with the different noise bands as the param¬ 
eter. The level of the speech was 95 db re 0.0002 dyne/cm 2 . 


Eight bands, continuous in frequency from 135 
to 4,000 c, were used to mask speech, and artic¬ 
ulation scores were obtained by the check-list 
method for several signal-to-noise ratios for 


pectation 3a and placed the maximum energy per 
cycle at approximately 500 c. The level per 
cycle can drop as much as 15 db between 1,000 
and 4,000 c. A comparison of this noise spec- 












118 


INTELLIGIBILITY OF SPEECH: TYPES OF INTERFERENCE 


trum with the long-time average spectrum for 
speech (see Figures 1 and 2 in Chapter 4) 
reveals a marked resemblance. The tentative 
generalization can be made, therefore, that the 
maximum interference with speech is produced 
by a noise spectrum which provides a constant 
speech-to-noise ratio at all frequencies. 


8 4 VOCAL INTERFERENCE 

The masking noise most apt to produce a 
constant speech-to-noise ratio at all frequencies 
is, of course, speech itself. Fortunately, how¬ 
ever, the analytical ability of the ear and the 
intermittent character of speech make it rela¬ 
tively simple to distinguish one voice from 
another. Serious interference results only when 
the desired voice is masked by a babble of other 
voices. 

Articulation tests using 1, 2, 4, 6, and 8 inter¬ 
fering voices showed that masking is greatest 
when 4 to 8 voices are used. A single voice is 
10 to 12 db less effective than a complex babble. 
The maximum masking obtained with vocal 
signals is equivalent to the masking obtained 
with the noise spectrum (described above) 
which produces the greatest loss in intelligi¬ 
bility. 

In general, the meaning of the interfering 
speech, and even the language, are not impor¬ 
tant factors in determining the masking effect. 
On the other hand, laughter and boisterous 
shouting are somewhat more distracting than 
conversational interference. Results using a 
babble of voices speaking Japanese were 
equivalent to results using an English babble. 
The conclusion is, therefore, that it is the spec¬ 
trum of the masking signal and not its meaning 


which produces the observed interference with 
intelligibility. 


8 3 ALTERN ATING INTERFERENCES 

In the course of the investigation of a wide 
variety of interfering signals, an adaptation 
effect was noticed for even experienced subjects 
when they were asked to listen for speech in 
the presence of some new and unfamiliar type 
of noise. Articulation scores rose as much as 
30 per cent during the first 15 minutes of 
testing with a new signal. Apparently the adap¬ 
tation effect results because the listener must 
decide what to listen to and what to ignore in 
the unfamiliar combination of speech and noise. 

These observations suggested the possibility 
that masking might be greater if the masking 
signal were changed from moment to moment. 
If the masking noise were changed before the 
listener could become adapted to it, the masking 
efficiency of the alternating signals might be 
increased above the average effectiveness of the 
component signals. An experiment was de¬ 
signed, therefore, to discover the effect of alter¬ 
nating the interference at several fixed intervals 
and at random intervals. 38 A slight effect in the 
expected direction was observed in the early 
stages of the experiment, but the effect disap¬ 
peared as the listeners grew accustomed to the 
alternations. The maximum effect, obtained 
with a random rate of alternation, was a shift 
of about 5 per cent in the articulation score. 
The listeners reported that the alternating sig¬ 
nals were much more distracting. In general, 
however, the effect of alternating the masking 
signals is negligible under the conditions of a 
laboratory test. 



Chapter 9 

THE INTERPHONE 


I N many WAYS the interphone stands as the 
prototype for speech communication sys¬ 
tems. The microphone, amplifier, and headset 
represent the indispensable elements: two elec¬ 
tro-acoustic transducers joined by an intermedi¬ 
ate transmitting network. The problems which 
arise are typical of the problems encountered 
in any communication system: intelligible trans¬ 


series of modifications of the original equip¬ 
ment. Even before the entry of the United 
States into World War II, persistent reports of 
the unsatisfactory performance of aircraft in¬ 
terphones were received. A program of testing 
in the field and in the laboratory was begun, 
and improvements soon became available in 
microphones, headsets, and amplifiers. Changes 


MICROPHONE 
AMPLIFIER 
HEADPHONES 
HELMET 
OXYGEN MASK 
COMPONENT CHANGES 


T-30-M T-30-M T-30-P(CLIP) T-30-P(CLIP) ANB-M-CI 

BC-347 BC-347-C BC-347-G BC-347-C BC-347-C 

R-14 R-14 ANB-H-I ANB-H-I 

B-6(WINTER) B-G(WINTER) B-6(WINTER) NAF 1092 

.A-14 A - 14 A-14 A- 14 



ANB-M-CI 
BC-347-C 
ANB-H-I ANB-H-I 

8-6 (WINTER) NAF 1092 
A-14 A-14 


BC-347-C 


T-30-P{CLIP) NAF 1092 
AND 

ANB-H-I 


ANB-M-CI 

AND 

8-6 (WINTER) 


NAF 1092 


Figure 1. Bar graph illustrating the improvement in interphone performance as old components were 
discarded and replaced by newer ones. 


mission of speech under a variety of operating 
conditions, exclusion of noise at all points in 
the system, and the maximum possible comfort 
and convenience for personnel. Thus the follow¬ 
ing discussion of aircraft interphones exempli¬ 
fies concepts and working principles which 
apply to a wide range of other equipment. 

The aircraft interphone in use at the end of 
World War II represented the culmination of a 


were adopted, communication was improved, 
and at the close of hostilities every major com¬ 
ponent of the original interphone had been re¬ 
placed. 

The story of these improvements is graph¬ 
ically told in Figure 1. This chart shows the 
averaged results of a series of articulation tests 
on six aircraft interphones. The tests were run 
with trained Service personnel at low altitude 


119 

















































120 


THE INTERPHONE 


(hatched) and at high altitude (unhatched) 
during flights in a B-17F bomber at Eglin Field, 
Florida. 5 Interphone No. 1, at the left of the 
chart, represents approximately the perform¬ 
ance of the original interphone. As the old com¬ 
ponents were discarded and replaced by newer 
ones, interphone performance improved steadily. 
The final version gave 60 to 70 per cent correct 
word articulation under operating conditions 
which had limited the earlier equipment to 10 
or 20 per cent, and further improvements were 
still under development. 

The value of these improvements can be 
measured in terms of greater military coordina¬ 
tion. The personnel of a combat vehicle must 
communicate with one another to operate 
smoothly as a unit, and they must maintain 
contact with distant points of command to op¬ 
erate as an effective component of a larger team. 
The interphone is the standard system for com¬ 
munication between members of the crew, and 
the effectiveness of the vehicle depends in large 
measure upon the effectiveness of the inter¬ 
phone. 


91 EVALUATION OF PERFORMANCE 
IN NOISE 

One of the important contributions of the 
Psycho-Acoustic and Electro-Acoustic Labora¬ 
tories in the early years of World War II was an 
insistence that the greatest single hazard to 
effective military communication is the presence 
of noise, and that interphone performance can 
be properly assessed only under the acoustic 
stress of ambient noise. Satisfactory perform¬ 
ance by an interphone system in quiet surround¬ 
ings does not insure satisfactory performance 
in noise. The evaluation of performance in noise 
became, therefore, a problem of major im¬ 
portance. 

In its simplest form, the interphone operates 
as shown in Figure 2. When a member of the 
crew wants to talk to another station, he de¬ 
presses his push-to-talk switch. The output volt¬ 
age from his microphone is then amplified by 
the interphone amplifier and heard in the ear¬ 
phones at all stations. In such a system there 
are three major ways in which noise can reach 
the listener’s ear (see Figure 3). 










Figure 2. Interphone system (schematic). 



Figure 3. Noise interference in a voice com¬ 
munication system. 


1. The microphone picks up a certain amount 
of the ambient noise N 1 around the talker. This 
noise reaches the listener via the interphone 
system. The noise spectrum reaching the 
listener may differ considerably from the spec¬ 
trum of the ambient noise N x . The noise spec¬ 
trum acting upon the microphone is a function 
of the ambient sound field, of the type and shape 
of microphone, and of the position of the micro¬ 
phone relative to the mouth and face of the 








































































EVALUATION OF PERFORMANCE IN NOISE 


121 


talker. The spectrum is further modified before 
reaching the listener by the overall transmis¬ 
sion characteristic of the interphone. 

2. Noise may enter the listener’s ear directly 
by leakage around the earphone and its cushion. 
This contribution depends on the noise spectrum 
N 2 at the listener’s location and on the noise- 
excluding properties of the earphone cushion. 
These properties are a function of the type of 
earphone and cushion, and of the particular 
way in which they fit the listener’s ear, and are 
independent of the rest of the system. 

3. Noise may also enter the system by way of 
the electric transmission system itself. This 
source of noise, usually negligible for inter¬ 
phones, becomes important in other communi¬ 
cation systems, e.g., a radio link. The noise 
spectrum at the listener’s ear due to this cause 
is, in part, a function of the transmission char¬ 
acteristics of the system. Noise entering the 
system when the push-to-talk switch at other 
stations is accidentally pressed and noise picked 
up by the earphones acting as microphones at 
other stations can be lumped, if desired, with 
the effects of the noise N 3 . 

It is clear that an appraisal of the perform¬ 
ance of interphones and their components must 
include not only their properties as electro¬ 
acoustic transducers for the speech signal but 
also the degree to which unwanted sound is 
excluded. 


911 Parameters Controlling Performance 

The capacity of an interphone system 3 to 
transmit speech intelligibly is measured by a 
count of the number of discrete speech units 
recognized by the listener. The percentage of 
units correctly perceived is called the articula¬ 
tion score (see Chapter 5). The problem, there¬ 
fore, is to correlate the efficiency of the inter¬ 
phone system, as measured by an articulation 
score, with its physical and psychophysical 
parameters. 

Two theories relating the intelligibility of 
speech to the parameters of the interphone have 
been proposed for linear systems by the Bell Tel- 

a Talker and listener must be considered integral 
parts of a voice communication system. 


ephone Laboratories. According to one theory, 1 - 9 
the ability of the listener to perceive and recog¬ 
nize speech sounds depends primarily upon the 
relative intensities of the speech and noise 
spectra at the listener’s ear, evaluated over 
most of the audible frequency range. 

The spectrum level [SL] (see Section 4.1) 
of the received speech is a function of many 
variables, the most important of which are: 

1. Voice level of the talker. 

2. Orthotelephonic gain of the interphone 
(see Section 7.2.1). 

3. Coupling between mouth and microphone. 

4. Coupling between ear and earphone. 

5. Speech material and composition. 

6. Enunciation and training of the talker. 

The spectrum level of the noise at the listen¬ 
er’s ear depends on: 

1. The ambient and electrical noise present. 

2. The noise-excluding properties of the cir¬ 
cuit, including microphone and earphone. 

3. The efficiency and electrical properties of 
the circuit. 

From this information and from certain as¬ 
sumptions and empirical relations derived from 
the fundamental properties of speech and hear¬ 
ing, an estimate can be made of the articulation 
scores to be expected. The estimates are most 
accurate when the spectrum of the noise at the 
listener’s ear extends over a wide range of fre¬ 
quencies, the noise levels are moderate, and the 
response characteristic of the system is free 
from abrupt changes with frequency. 

The correct calculation of scores on an abso¬ 
lute basis is difficult and of little practical mean¬ 
ing in view of the many variables involved. 
Comparison of measured and computed articu¬ 
lation scores on a relative basis, however, has 
yielded some useful results. A very desirable 
feature, from the design engineer’s point of 
view, is that estimates can be made of the effects 
on the performance of the system of variations 
of circuit constants and elements. Predictions of 
performance based on this theory are considered 
in detail in Section 9.1.5. 

The second theory 10 assumes that the differ¬ 
ential sensitivity of the ear to frequency and 
level (see Section 3.5) is the controlling factor 
in the successful interpretation of speech 
sounds. Empirical functions were obtained at 




122 


THE INTERPHONE 


the Bell Telephone Laboratories relating the 
speech spectrum, the differential sensitivity of 
the ear, the masking of sounds, and the articu¬ 
lation score. No use is made of this theory in the 
work reported here. 

The important contribution of the two avail¬ 
able theories is the demonstration that, in the 
simplest cases, the overall response of the sys¬ 
tem and its noise-excluding properties are of 
fundamental importance in determining per¬ 
formance. By assuming “average normal” talk¬ 
ers and listeners, standardized and uniform 
speech material, “constant” voice level, and 
negligible interaction between mouth and mi¬ 
crophone, many of the troublesome variables 
can be effectively eliminated. However, this 
simplification, while useful for theoretical anal¬ 
ysis, does not extend to many practical condi¬ 
tions of use. Hence, in evaluating interphone 
performance, articulation tests under simulated 
conditions of use should be performed wherever 
feasible. 


912 Overall Response of the System 

In the preceding section it was stated that 
systems can generally be rank-ordered as to 
performance under certain simplified conditions 
if the levels of received speech and noise at 
the listener’s ear are known as a function of 
frequency. In order to obtain this information, 
it is necessary to consider the overall response 
of the system. 

Meaningful comparisons of the overall fre¬ 
quency response of two systems can be most 
readily obtained by defining a suitable refer¬ 
ence system. The orthotelephonic reference sys¬ 
tem (see Section 7.2.1) consists of a “normal” 
talker and listener 1 * facing each other at a 
distance of 1 m in a quiet, nonreverberant 
room. The orthotelephonic gain [OG] at a given 
frequency is then expressed as the ratio of the 
level produced at the listener’s ear by the sys¬ 
tem under test to that at the listener’s ear when 
speech is transmitted from talker to listener 
over the 1-m path in free air. It is assumed, 

b This is in contrast to methods using artificial voices 
and ears. The chief merit of using the latter lies in the 
simplicity achieved (see Chapter 10). 


of course, that the talker speaks into the system 
under test in the same way he talks over the 
orthotelephonic reference system. By definition, 
then, speech heard over a system having an 
OG of 0 db at all frequencies will sound the 
same as speech heard over an air path 1 m in 
length. 0 

In order to obtain a quantitative measure of 
OG, it is necessary first to decide how the level 
at the listener’s ear is to be measured. One 
method involves two measurements of the sound 
pressure at the listener’s eardrum: first when 
he listens to the system under test and second 
when he listens to the talker over a direct air 
path 1 m in length. The ratio of these two 
pressures, expressed in decibels, is a direct 
measure of the orthotelephonic gain of the 
system. 14 

In practice, however, this measurement 
would be very difficult to perform. An alterna¬ 
tive procedure, and one which yields the same 
results, involves the subjective loudness pro¬ 
duced by a given sound source expressed in 
terms of the equivalent free-field sound pres¬ 
sure. The equivalent free-field pressure is ob¬ 
tained by requiring an observer located in a 
nonreverberant, quiet room to equate the loud¬ 
ness produced by a sound source some distance 
in front of him with that produced by the ear¬ 
phone. The equivalent free-field pressure is then 
defined as the free-field pressure, measured at 
the listener’s location, which produces the same 
loudness as does the earphone under test. Spe¬ 
cifically, by free-field sound pressure is meant 
here the pressure measured by a “point” micro¬ 
phone located at the position where the listener 
later places his head when making the loudness 
balance. This location is taken as the mid-point 
of a line joining the listener’s ears. 

The concept of equivalent free-field pressure, 
introduced by the Bell Telephone Laboratories, 
has the advantage that measurement of the 
sound pressure is more easily performed in the 
free field than at the listener’s eardrum. Al¬ 
though there is some evidence to the contrary 

c If the experiment is actually performed in which a 
listener compares what he hears over a system with 
0-db OG with what he hears over an air path of 1 m, 
certain differences, for example, in apparent localiza¬ 
tion may be experienced. Such differences are disre¬ 
garded here. 





EVALUATION OF PERFORMANCE IN NOISE 


123 


(see Sections 3.2 and 10.3), it will be assumed 
here that for a given frequency and observer 
there is an essentially constant relation between 
the pressure at the eardrum and the corre¬ 
sponding equivalent free-field pressure for vari¬ 
ous types of earphones and pressure levels (see 
Section 3.1). 

Consequently, the orthotelephonic gain of a 
system (in decibels) can be defined for any 
given frequency as follows: 

OG = 20 log (1) 

Po 

This equation can be expressed in words. The 
OG of a system X is the ratio, in decibels, of 
the equivalent free-field pressure p x , produced 
by system X, to the free-field pressure p 0 , gen¬ 
erated 1 m from the talker in a quiet, non- 
reverberant room. Since the talker must speak 
the same way d into both system X and the 
orthotelephonic system, the OG of the system 
is independent of the intensity and spectrum of 
the talker’s voice. 

If the OG of a system is known as a function 
of frequency, the speech spectrum heard by 
the listener is easily obtained. First, the spec¬ 
trum of the talker’s voice is determined in 
terms of free-field pressure p 0 , at 1 m in an 
anechoic (echo-free) chamber. Average speech 
spectra of this type were given in Figures 1 
and 2 in Chapter 4. (They are easily reducible 
to a distance of 1 m.) Second, this speech spec¬ 
trum is added (in terms of decibels) to the OG 
of the system X. The result is the speech spec¬ 
trum heard by the listener over system X, 
expressed in terms of the equivalent free-field 
sound pressure p x . 


913 Synthesis of the Overall Response 

In practice, it is convenient to synthesize the 
overall response of the system in three steps. 
The response of the three basic components of 
the interphone system—the microphone, the 
earphone, and the network connecting these 


d Difficulties in keeping the voice “constant” are 
encountered if marked interaction exists between the 
talker and the microphone he uses, as, for example, 
when an oxygen mask is used. 


two electro-acoustic transducers—is determined 
separately. These three response measurements, 
if properly made, can be added to obtain OG. 

This procedure may best be illustrated by 
reconsidering equation (1) above. This equa¬ 
tion can be rewritten in terms of three factors, 
as follows: 

OG =20 log *-20 log (^) (f)(|), 

= 20 log — + 20 log - + 20 log (2) 

The three terms of equation (2) can be identi¬ 
fied with the response characteristics of the 
three elements of the system in the following 
way. 

Microphone. The term 20 log e/p 0 is the ratio, 
in decibels, of the voltage developed by the 
microphone, held in its normal relation to the 
talker, to the free-field sound pressure measured 
1 m away from the talker with the microphone 
removed. An audio-spectrometer (see Chapter 
4) is used to measure e and p 0 in suitable con¬ 
tiguous frequency bands. The measurement of 
p 0 is made under orthotelephonic reference con¬ 
ditions. The quantity 20 log e/p 0 may be re¬ 
ferred to as the real-voice response of the 
microphone. 

Amplifier. The term 20 log E/e is the ratio, 
in decibels, of the voltage E developed across 
the listener’s headset to the voltage e developed 
by the microphone. This term is simply the 
electrical response of the network connecting 
the two electro-acoustic transducers. 

Earphone. The term 20 log pJE is the ratio 
of the equivalent free-field sound pressure p x to 
the voltage E measured across the terminals of 
the earphone. More specifically, p x is the free- 
field pressure necessary to produce the same 
loudness sensation as that produced when a 
voltage E is applied to the earphone. In making 
the loudness-balance judgment the listener is 
located in a quiet, nonreverberant room with 
the sound source in front of him. It is not nec¬ 
essary to use a speech signal. The quantity 
20 log pJE may be referred to as the real-ear 
response of the earphone. 

Addition of these three terms gives the ortho¬ 
telephonic gain for the system at any given 
frequency. 




124 


THE INTERPHONE 


9-1-4 Response of the System to Noise 

The level and spectrum of the noise at the 
listener’s ear is the sum of the contributions 
from the noise transmitted by the microphone, 
the noise reaching the listener’s ear through 
lack of insulation of the earphone cushion, and 
any contributions from noise due to other causes 
(see Figure 3). For noise of the type found in 
combat vehicles (see Chapter 2), the resultant 
noise spectrum at the listener’s ear is obtained 
by summation (on a power basis) of the vari¬ 
ous contributions. 

The noise entering the system through the 
microphone can be measured readily by using 
an audio-spectrometer (see Section 10.1.2). A 
“talker” (who, however, does not talk) holds 
or wears the microphone under test in a diffuse 
sound field corresponding to N 1 (see Figure 3), 
while the voltage developed by the microphone 
due to noise pickup is measured. By addition 
of the amplifier response and real-ear response, 
the equivalent free-field spectrum level of the 
noise due to N t is obtained. 

To obtain the noise spectrum leaking through 
the earphone cushion, the listener might be im¬ 
mersed in the diffuse noise field N 2 wearing the 
earphones under test in normal conditions of 
use, and a measurement made of the noise 
spectrum at the eardrum would yield the de¬ 
sired results. A much simpler method deter¬ 
mines the attenuation which the headset pro¬ 
vides for pure tones (see Section 10.3.2). The 
attenuation, in decibels, is then subtracted from 
the noise spectrum N,, and the spectrum reach¬ 
ing the listener’s ear through the cushion is 
thus expressed approximately in terms of equiv¬ 
alent free-field sound pressures. 

Still another method consists in measuring 
the masked threshold of the listener for pure 
tones when noise is leaking through the cushion. 
This is accomplished by energizing the ear¬ 
phone with single frequencies. This method is, 
in many ways, the most fundamental one, in 
that it gives a direct measure of the reduced 
auditory area available for communication. It 
takes into account such effects as masking by 
adjacent noise components (spread of mask¬ 
ing). It can be used to test the noise exclusion 
properties of earphones (see Section 10.3.2), 


but is an even more valuable laboratory method 
if applied to the total noise spectrum at the 
listener’s ear due to N u N 2 , and 2V 3 . 

The contribution due to electrical noise N 3 
is evaluated by an analysis of the voltage ap¬ 
pearing across the earphone terminals. Addi¬ 
tion of this spectrum to the real-ear response 
of the earphone yields the desired result. 


9-1-5 Prediction of Performance 

Now that we have considered in detail the 
procedures for determining the speech and 
noise spectra at the listener’s ear, we can pro¬ 
ceed to demonstrate how this information is 
used to predict the performance of an inter¬ 
phone system. The following computational 
procedures were devised originally by the Bell 
Telephone Laboratories 12 and later refined in 
some respects by the Electro-Acoustic Labora¬ 
tory. 142 

It is the fundamental assumption of the 
theory that the contribution to intelligibility 
by any given band of speech frequencies may 
be calculated independent of the contributions 
made by other bands. These contributions can 
be expressed by means of an empirical function, 
the articulation index [AI], which has additive 
properties. 6 The total contribution to AI over a 
total range of frequencies is the sum of the 
contributions to AI of the component bands 
contained in that range, essentially independent 
of the manner of subdivision. 

The sum of the contributions of all the bands 
of frequencies passed by the interphone can be 
written as follows: 

AI - 2 (AAI)„, (3) 

p=l 

where (AAI) P is the contribution to the articu¬ 
lation index of the pth band of speech frequen¬ 
cies in the total range under consideration. 
Clearly, (AAI) p is a function of both the fre¬ 
quency limits of the band and of the speech 
and noise levels in that band. 


e It is also assumed that there is a unique relation 
between articulation score and AI for a given speech 
material and crew. 




EVALUATION OF PERFORMANCE IN NOISE 


125 


The problem, of course, is to design the inter¬ 
phone in such a way as to maximize AI, i.e., to 
maximize the contribution from each of the p 
bands of frequencies. It is plausible to assume 
that this condition will obtain for low levels of 
noise and high levels of speech, and (AAI) p max 
will then depend primarily upon frequency. 

We therefore put 

(AAI)p = (AAI)r W p . (4) 

The weighting factor W p can be interpreted 
as the fractional part of (AAI) p max which is 
available when the speech and noise levels are 
unfavorable. Thus W p is primarily a function 
of the speech-to-noise ratio in the pth band, 
and it is convenient to assume that it is inde¬ 
pendent of frequency. Hence, 

, L~," max 

AI = £ (AAI)p W p . (5) 

p=i 

It is necessary, therefore, to evaluate (AAI) p max 
experimentally as a function of frequency and 
W p as a function of the signal-to-noise ratio. 

Relation between (AAI) p max and Frequency 

This relation has been evaluated at the Bell 
Telephone Laboratories for nonsense syllables 


100 1000 10,000 
2 3 4 56769 2 3 456789 



Figure 4. Articulation scores vs cutoff fre¬ 
quency of high-pass and low-pass filters inserted 
in a wide-band system. (BTL) 


with both men and women talking. Articulation 
tests were made in quiet on a high-fidelity 
system into which low-pass and high-pass filters 


could be inserted. The results of these tests are 
presented in Figure 17 of Chapter 7. 

If the optimal scores for each filter condition 
are plotted as a function of filter-cutoff fre¬ 
quency, the upper curves in Figure 4 result. It is 
evident from these curves that as much of the 
total intelligibility is carried by frequencies 
below 1,900 c as is carried by the frequencies 
above 1,900 c. Furthermore, the first point on 
the additive AI scale, the 50 per cent point, is 
thus determined. It appears that 50 per cent 
AI corresponds to an articulation score of 68 
per cent. Another point can be obtained by 
returning to the orthotelephonic gain of the 
wide-band system which gives an articulation 
score of 68 per cent. From Figure 17, Chapter 7, 
we see that the wide-band system gives 68 per 
cent articulation with an orthotelephonic gain 
of about —31 db. If the articulation scores are 
now plotted as a function of filter-cutoff fre¬ 
quencies for this gain setting, the lower curves 



ARTICULATION INDEX, PERCENT 

Figure 5. Approximate relation between sylla¬ 
ble articulation and articulation index. (BTL) 


in Figure 4 result. These functions cross at a 
point which defines an AI of 25 per cent. 

By a similar fractionating procedure, the com¬ 
plete function relating AI to syllable articula¬ 
tion score can be plotted. The function has the 
general shape shown in Figure 5. For different 
speech material and crews, other curves would 



























126 


THE INTERPHONE 


have been obtained. By means of this function, 
the ordinate in Figure 4 can be converted di¬ 
rectly into AI. This has been done in Figure 6, 
where the cumulative contribution to the artic- 



Figure 6. Relating articulation index and cut¬ 
off frequency of a low-pass filter inserted in a 
wide-band system. (BTL) 


ulation index is shown as a function of the 
upper cutoff frequency. The slope of this func¬ 
tion shows the relative importance of the vari¬ 
ous frequencies in their contribution to intelli¬ 
gibility. The similarity between this function 
(see Figure 6), the pitch scale (see Figure 7, 
Chapter 3), and the function which relates the 
position of maximum agitation on the basilar 
membrane to the frequency of the acoustic 
stimulus has been pointed out (see Section 
3.3.1). 

With this information, the range of impor¬ 
tant speech frequencies can be divided into a 
number of bands which contribute equally to 
the articulation index. Computations are nor¬ 
mally made on the basis of 20 bands, each con¬ 
tributing 5 per cent. 

Relation between W p and the Levels of 
Speech and Noise 

As the simplest case, consider first the com¬ 
putation of W p for quiet conditions. If the 
entire frequency range of speech is divided 
into 20 bands, each contributing 5 per cent to 
AI, then (AAI) p can be plotted between 0 and 
5 per cent as a function of the orthotelephonic 
gain for each of the 20 bands. When this is 


done, it is found that W p depends principally 
upon the sensation level of the speech in the 
band, i.e., upon the amount by which the speech 
level exceeds the threshold of hearing in the 
band. 

These results can be carried over directly to 
the case where noise is present if it is assumed 
that we are dealing with the masked threshold 
instead of the quiet threshold of hearing. Now 
we have previously seen (see Section 3.4.1) that 
the masking of pure tones by noise is directly 
proportional to the effective level Z of the band 
of noise frequencies (see Figure 10, Chapter 
3). Hence W p can be expressed directly in terms 
of the speech-to-noise ratio in each band. The 
spectrum level of the noise B in decibels is 
subtracted from the spectrum level of the 
speech B s in decibels. Because the temporal 
distribution of amplitudes differs for speech 
and noise, a constant number of decibels is 
added to the speech-to-noise ratio (£ s - B) in 
order to fit the experimental evidence. 115 



b s /bin db 

SPEECH-TO-NOISE RATIO 

Figure 7. Weighting factor vs speech-to-noise 
ratio. (BTL) 

Figure 7 shows approximately how the 
weighting factor W v depends upon the speech- 
to-noise ratio B x - B. If the signal-to-noise ratios 
have been determined for each of the 20 bands 
of speech frequencies, the corresponding values 
for W p can be read from Figure 7. It can be 
seen that the speech-to-noise ratio must be of 
































































INTERPHONE COMPONENTS: THE MICROPHONE 


127 


the order of 20 to 25 db before a band of speech 
frequencies makes its maximum contribution to 
the articulation index. 

In making calculations, a work sheet like the 
one shown in Figure 8 may be helpful. The 
frequency scale is divided into 20 bands, each 
having a maximum potential contribution to 


spectrum at the listener’s ear is shown in 
Figure 10, expressed in terms of the equivalent 
free-field spectrum level. While, in some in¬ 
stances, the agreement between the experiment 
and the calculations is not especially close, the 
theory seems to provide a workable estimate 
of the relative performance of the systems. 



LOWER--► 200 330 430 560 700 840 1000 1150 1310 1480 1660 1830 2020 2240 2500,2820 3200 3650 4250 5050 

UPPER-► 330 430 560 700 840 1000 1150 1310 1480 1660 1830 2020 2240 2500 2820 3200 3650 4250 5050 6100 

MEAN -*■ 270 380 490 630 770 920 1070 1230 1400 1570 1740 1920 2130 2370 2660 3000 3400 3950 4650 5600 

BAND NO.—>■ I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 

FREQUENCY BANDS OF EQUAL CONTRIBUTION TO ARTICULATION INDEX 
Figure 8. Work sheet for evaluating the articulation index of a communication system with noise present. 


AI of 5 per cent. The weighting factor can then 
be obtained from Figure 7 for each band. Addi¬ 
tion of all contributions to AI according to 
equation (5) determines the total articulation 
index. If it is desired, once AI is known, the 
syllable articulation score can be predicted 
from Figure 5. In this manner, an approxima¬ 
tion of the interphone’s performance is obtained 
solely on the basis of the speech-to-noise ratio 
at the listener’s ear/ 

Figure 9 shows a comparison of theory and 
experiment. The system under test was a band¬ 
pass system of variable bandwidth. The noise 


f Refinements have been added to the theory, taking 
into account such secondary variables as interband 
masking, self-masking, and nonlinear distortion of cer¬ 
tain types. 


9.2 INTERPHONE COMPONENTS: 

THE MICROPHONE 

The task of delivering a signal at a suitable 
speech-to-noise ratio over a wide band of fre¬ 
quencies is the joint responsibility of the micro¬ 
phone, the amplifier, and the headset. Failure 
on the part of any one of these components 
means failure of the entire interphone. 

The first link in the chain is, of course, the 
microphone. Several different types of micro¬ 
phones were used in World War II: hand¬ 
held microphones, noise-canceling microphones, 
throat microphones, microphones mounted in 
enclosures. Each type was represented by a 
variety of models, and it is necessary, therefore, 
to limit the following discussion to those micro- 




























































128 


THE INTERPHONE 


phones which were actually used on a large 
scale and to the general factors governing the 
performance of each of the generic types. 


921 Hand-Held Microphones 

The two hand-held microphones most widely 
used in aircraft were the Army’s T-17 and the 
Navy’s T-38 (RS-38-A). Photographs of these 



Figure 9. Measured and computed gain func¬ 
tions for band-pass systems in noise. 


two microphones are shown in Figures 11 and 
12. Both the T-17 and the T-38 are carbon- 
button instruments. 



Figure 10. Noise spectrum at the listener’s ear 
in articulation tests described by Figure 9. 


Carbon-type microphones were adopted al¬ 
most exclusively by the Armed Forces of the 
United States, despite the fact that all carbon 
microphones are essentially nonlinear devices. 
Furthermore, their frequency-response charac¬ 


teristic depends upon the level of the sound 
pressure for which the response is obtained. 
They require a separate source of current, their 
performance is likely to vary with change of 
position, and they produce a certain amount of 
“burning noise” due to the passage of current 
through the carbon granules. In addition, there 
is some deterioration of the carbon granules 
with age. Their extensive use is due primarily 
to their very high sensitivity, far exceeding the 
sensitivity of magnetic and dynamic micro¬ 
phones. In spite of their limitations, the T-17 
and T-38 microphones proved to be reliable 
instruments for communication. 

Hand-held microphones as a class have cer¬ 
tain disadvantages which limit their usefulness. 
They do not exclude ambient noise as well as 
the other types of microphones, and they re¬ 
quire the continuous use of one hand during 
communication. 

Measurements of the speech-to-noise ratio 
were made for these and other microphones. 
Eleven sets of highly discriminative band-pass 
filters, having cutoff frequencies indicated on 
the figures, were used for the analyses. A talker 
held the microphone close to his mouth in an 
ambient noise field similar to that found in two- 
engined aircraft without sound treatment. The 
talker remained silent while the noise level 
from the microphone was measured for each 
of the pass bands. The noise was then turned 
off, and the analysis repeated as the talker 
spoke the sentence, “Joe took father’s shoe- 
bench out; she was waiting at my lawn.” The 
results of these measurements are presented in 
Figures 13 and 14. The speech-to-noise ratio in 
any band is found by counting the number of 
decibels between the levels of the two signals. 

Measurements of this type are influenced by 
a variety of variables and therefore should 
never be regarded as absolute. Their value lies 
in the fact that they provide a basis for com¬ 
paring different microphones all tested under 
identical conditions. Comparison of the T-17 
and T-38 shows that the speech-to-noise ratios 
are nearly the same at all frequencies. This 
agrees with the usual finding that the two 
microphones yield approximately equivalent ar¬ 
ticulation scores when tested under the same 
conditions. 









































INTERPHONE COMPONENTS: THE MICROPHONE 


129 




Figure 11. Hand-held carbon microphone T-17. 


Figure 12. Hand-held carbon microphone T-38. 




100 1000 10,000 
FREQUENCY IN CYCLES PER SECONO 


Figure 13. Speech and noise levels obtained for 
different frequency bands with the T-17 micro¬ 
phone. 


Figure 14. Speech and noise levels obtained for 
different frequency bands with the T-38 micro¬ 
phone. 





































































130 


THE INTERPHONE 


9-2-2 Noise-Canceling Microphones 

Noise-canceling, or differential, microphones 
have both sides of the diaphragm exposed to 
the impinging sound waves and are, therefore, 
sensitive to the pressure gradient in the sound 
field. When the differential microphone is close 
to and oriented toward the talker’s lips, it is 
activated primarily by the talker’s voice and 
not by the noise field. 




Figure 15. Noise-canceling T-45 microphone. 

The noise-canceling microphone, T-45, used 
by the Army Ground Forces, is shown in Fig¬ 
ure 15. A later improved version, M-5/UR, is 
shown in Figure 16. Because of the manner of 
suspension in front of the talker’s lips, these 
were sometimes called “moustache” or “lip” 
microphones. This suspension leaves the hands 
free (if a suitable switch is provided), and thus 


has one great advantage over the hand-held 
microphone. 

Speech and noise measurements for noise¬ 
canceling microphones, comparable to those 
presented for hand-held microphones, are 
shown in Figures 17 and 18. It can be seen 
from these figures that the speech-to-noise ratio 
is most favorable at frequencies below about 
2,000 c. In this region the ratio is somewhat 
better than that of the hand-held microphone, 
although neither microphone performs very 
poorly here. Unfortunately, the noise-canceling 
microphone possesses an inherent limitation in 
its inability to discriminate against high-fre¬ 
quency noise, and in high ambient noise this 



Figure 16. Noise-canceling M-5/UR microphone. 


becomes a serious consideration. 13 It is usually 
in the region above 3,000 c that noise levels 
actually exceed speech levels, and in this region 
the noise canceler is just another microphone. 

Figures 17 and 18 also include measurements 
of “wind noise.” The talker wore the micro¬ 
phone in a 30-mph wind and adjusted the 
position of his head until the maximum level 
of noise was obtained. He then held this posi¬ 
tion, and the noise due to the wind was meas¬ 
ured in each frequency band. The results show 
that the T-45 is more susceptible to wind noise 
than the M-5/UR, and this was confirmed by 
the results of articulation tests. In both cases, 





















INTERPHONE COMPONENTS: THE MICROPHONE 


131 















10 4 


SPEECH 


















> 







AMBIENT NOISE 

i 


...O'' 




\ 

N 

s \ 

A 

\ 






























MICNOPMONE NOISE CANCCLII 
TYNE T-48 

«G 

1 











••o rw io 

. i i i . i , i ,i.i, 


B B 


” iTuiTui *., 


«* » J 4 5 * 7 •• ? S 4 5 I 7I» 

*00 1000 10,000 
frequency in cycles per secono 


Figure 17. Speech and noise levels obtained for 
different frequency bands with the T-45 micro¬ 
phone. 


however, the noise-canceling microphones gen¬ 
erate a higher overall noise level in this wind 
than they do when used in an ambient noise of 
120 db. 

While the noise-canceling microphones possess 
limitations which cannot be overcome, they 
proved to be the most satisfactory devices avail¬ 
able in moderate or low noise levels or in in¬ 
stances where acoustic feedback was a critical 
problem. It was a real misfortune that the 
sensitivity of aviators’ lips, or their fondness 
for moustaches, and the relative lack of sensi¬ 
tivity of their necks should have delayed the 
substitution of this microphone for the ubiq¬ 
uitous throat microphone. 



• 9 2 3496789 2 3496789 

100 1000 10,000 


FREQUENCY IN CYCLES PER SECONIT 


Figure 18. Speech and noise levels obtained for 
different frequency bands with the M-5/UR 
microphone. 



9-2-3 Throat Microphones 

A device used widely by the U. S. Army Air 
Forces at the beginning of World War II was 
the throat microphone. In this assembly the 
microphone is strapped to the throat directly 
above the larynx. Such an arrangement pos¬ 
sessed the advantages of apparently low noise 
pickup and free use of hands, and it would 
probably have been a very effective instrument 
but for the fact that the speech signal available 
at the larynx is intrinsically unintelligible. A 
photograph of the standard throat microphone, 
T-30, is shown in Figure 19. 

Articulation tests showed persistently that 
the “mushy” speech picked up by a throat 
microphone provides a poor means of com- 



Figure 19. Throat microphone T-30. 

























































132 


THE INTERPHONE 


munication. That this inferiority was not due 
to poor design of the American version is illus¬ 
trated by the articulation results shown in 
Figures 20 and 21. These figures present the 



Figure 20. Articulation as a function of the 
voltage amplification applied to the outputs of 
six different microphones. 


results of articulation tests run in 120 db of 
airplane noise with three hand-held and three 
throat microphones. 11 The Japanese and the 
British (ZA 13935) throat microphones are 


instrument consisting of the dynamic earphone 
ANB-H-1A used as a microphone. Figure 20 
shows the articulation of each of these micro¬ 
phones plotted as a function of the voltage gain 
of the interphone amplifier. The two carbon 
microphones, T-17 and T-30, are clearly the 
most sensitive and require the least amplifica¬ 
tion, while the two magnetic instruments, the 
British and Japanese throat microphones, are 
the least sensitive. If these functions are now 
replotted according to the level of the speech 
received at the listener’s ears, they group as 
shown in Figure 21. The three hand-held micro¬ 
phones are clearly superior to the three throat 
microphones. 

This inferiority of the throat microphone 
was repeatedly demonstrated by tests in the 
laboratory and in the field, in spite of the fact 
that the overall noise levels seemed favorable. 
Figure 22 shows the speech and noise levels 
comparable to those presented above. It will be 
noted that the speech level falls rapidly above 
1,500 c and this correlates with the “mushy,” 



SPEECH VOLTAGE ACROSS HEADSET (IN D8 RE .775 VOLT) 

Figure 21. Word articulation provided by six microphones when the amplification used with each micro¬ 
phone was adjusted to produce approximately the same speech level at the listeners’ ears. Each point 
is based on 200 words. 


both magnetic instruments; the American T-30 
is of the carbon type. The hand-held microphone 
was represented by the American T-17. The 
measurements included the Canadian dynamic 
ZA/CAN 5155 in a noise shield, and a special 


consonantless quality of the signal. In this in¬ 
stance, therefore, the usual advantage of a 
positive speech-to-noise ratio is of little value 
because the speech itself is so unnatural. 

The second merit of the throat microphone, 











INTERPHONE COMPONENTS: THE MICROPHONE 


1S3 


the freedom of the hands, also proved illusory 
in practice. The position of the microphone was 
critical and required frequent adjustment, and 
talkers who were frustrated by the poor per¬ 
formance of the instrument would often use 


>04 














—-'■-S4CCCM 












>0 4 






\ 









V . 




^ i'— 

\ 

\ 













\ 

\ 













\ 





MICROPHONE T-5( 

r. 











cororr 5535 


300 . ? 




■ 


Ti... 


I i l. . . . , i ■ i ■ i ■ i ■ i.i.i . . . _____ . i ... 

8 9 Z 5 4 5 6 7 0 9 Z 5456789 


100 1000 10,000 
FREQUENCY IN CYCLES PER SECONO 

Figure 22. Speech and noise levels obtained for 
different frequency bands with the T-30 micro¬ 
phone. 

one hand to press the microphone against their 
throats. All in all, the results with the throat 
microphone were not what had been hoped, 
and its belated replacement by other types of 
microphones was one of the major improve¬ 
ments made in the interphone system. 


9 . 2.4 Microphones Mounted in an Enclosure 

Mounting a small microphone inside an oxy¬ 
gen mask or a noise shield combines the ad¬ 
vantages of noise exclusion with free use of 
the hands. The effect of such an arrangement, 
however, is to reinforce the low frequencies, 
and the speech acquires an unfortunate, “boom¬ 
ing” character. One of the fundamental require¬ 
ments, therefore, is that the mask microphone 
should have a poor low-frequency response in 
order to make the response of the mask and 
microphone combination approximately uni¬ 
form. The fidelity and level of speech trans¬ 
mitted over these devices depends to a large 
extent upon the acoustic properties of the enclo¬ 
sure, and the interaction between the response 
characteristics of the mask and the microphone 
poses a very difficult problem for analysis. In 


most cases an empirical evaluation is necessary. 

The mask microphone adopted by both the 
Army and Navy was a carbon instrument, 
ANB-M-C1. A magnetic mask microphone, 
MC-253-A, was tested extensively and was 
adopted for use by the British. A photograph 
of these two microphones is shown in Figure 23. 



Figure 23. Mask microphones ANB-M-C1 (car¬ 
bon) and MC-253-A (magnetic). 


As in the case of the throat microphone, the 
interaction between the mask microphone and 
the voice precludes the simplifying theoretical 
assumptions made in Section 9.1.5. The articu¬ 
lation efficiency which can be realized with a 
properly designed mask microphone is consid¬ 
erably better than that obtained with throat 
microphones. When oxygen masks must be used, 
therefore, it is recommended that the micro¬ 
phone be placed inside the mask rather than on 
the throat. A photograph of the standard oxy¬ 
gen mask, A-14, used by both the Army and 
Navy, is shown in Figure 24. The microphone 
is mounted in the mask just above the outlet 
valve. 

There are also some advantages to be gained 
at low altitudes by mounting the microphone 
in an enclosure. Noise shields were developed 
which provided a good speech-to-noise ratio, 
eliminated wind noise, and left the hands free. 
A photograph of the ANB-M-C1 microphone 
mounted in the Harvard D-17 noise shield is 
shown in Figure 25. Speech, ambient noise, and 
wind noise measurements are shown in Figure 
26. The speech-to-noise ratio is excellent at all 
frequencies, and the effect of a 30-mph wind 
is much less serious than in the case of the 
noise-canceling microphones (see Figures 17 
and 18). A noise shield was also developed by 
the Bell Telephone Laboratories which covered 
the nose and the mouth and which performed 
even better than the Harvard D-17 shield. 






























134 


THE INTERPHONE 


Neither of these shields was adopted by the 
Services, however. 

An interesting comparison of the four types 
of microphones is given in Figure 27. 7 Articu¬ 
lation tests were run in ambient noise similar 
to that encountered in the cabin of a two- 
engined aircraft without sound treatment. 
Speech levels were held constant throughout all 
the tests. Five of the six microphones were 
nearly equivalent; the throat microphone T-30 




Figure 24. Oxygen mask A-14. 


From such comparisons it appears that the 
noise shield has much to recommend it. Noise 
shields are not, however, as comfortable as 
some of the other classes of microphones and, 
in certain situations, they might restrict the 
useful field of vision. Also, they would have to 
be issued individually, whereas the noise-can¬ 
celing microphone does not. These considera¬ 
tions probably contributed to the adoption of 
noise-canceling microphones rather than noise 
shields for use at low altitudes. 



Figure 25. Noise shield D-17. 


was strikingly inferior. Best results were ob¬ 
tained with the ANB-M-C1 in a noise shield, 
but the superiority over the noise-canceling 
microphones was small. The hand-held T-17 
ranks fifth in this group. 


93 INTERPHONE COMPONENTS: 

THE AMPLIFIER 

The second link in the chain from talker to 
listener is the interphone amplifier. The re¬ 
quirements for a satisfactory amplifier are 





















INTERPHONE COMPONENTS: THE HEADSET 


135 


straightforward and can be easily met by care¬ 
ful engineering: uniform response, sufficient 
voltage gain, and a power output adequate for 
the number of headsets used. The problem of 


® O A 














SPEECH ^ 













AMBIENT NOISE 












®0A 

BIB? "" 

V" 

.--o'' 



V' 


A 

A; 













V, 










V 

✓ 

A 

■> 

V 




MICROPHONE ANB 

J 

NOISE SMH 
1 

ELD 






N 

\ 



FIIUR COTOFFS I 

1 1 ... .11 

60 >• 

■4.4. ...1 . 

M «7 

O .04 


K * 


v. v 

■o 6000 *1 

I 1 1 1 

jJLl 


” ■ 4 ■ 


89 * 3456789 2 3456789 

too iooo 10,000 


FREQUENCY IN CYCLES PER SECOND 

Figure 26. Speech and noise levels obtained for 
different frequency bands with the ANB-M-C1 in 
the D-17 noise shield. 

noise exclusion, so serious for microphones 
and headsets, is not a factor in the design of 
the amplifier. 

The interphone amplifier BC-212, with which 



VOLTAGE GAIN OF INTERPHONE IN DB 


Figure 27. Word articulation as a function of 
voltage gain for six carbon microphones. 

the AAF began World War II, satisfied none 
of the fundamental requirements for satisfac¬ 
tory performance, and it was soon replaced by 
a more adequate unit, BC-347C. This amplifier 
possessed a satisfactory frequency response and 


was fairly adequate for communication at low 
altitudes. At high altitudes (see Section 9.5) 
there is a marked decrease in headphone sensi¬ 
tivity and in the voice level which a talker is 
able to maintain. Extensive flight tests at Eglin 
Field 0 showed that at altitudes of 35,000 ft 
neither the voltage nor the power-output ca¬ 
pacity of the amplifier BC-347C were adequate. 
As a result of these tests, design recommenda¬ 
tions were formulated. The frequency response 
at sea level should be essentially flat between 
300 and 4,000 c, with an overall voltage gain of 
approximately 18 db. At 35,000 ft the overall 
gain should be increased to approximately 30 
db, and the amplifier should be capable of de¬ 
livering a peak power of at least 200 milliwatts 
per headset, preferably 400. 

Following these recommendations, the inter¬ 
phone amplifier AM-26/AIC was adopted by 
the Army Air Forces. This amplifier possesses 
a power-output capability of approximately 5 
watts as compared to approximately 0.75 watt 
available from the amplifier BC-347C which it 
replaced. In addition, the AM-26/AIC provides 
increasing voltage gain with altitude. 

The improvements in the amplifier, there¬ 
fore, were all changes in the direction of more 
uniform frequency response and more adequate 
output power. Once the minimum requirements 
had been clearly demonstrated by experimental 
tests, most of the difficulties were readily over¬ 
come. 


94 INTERPHONE COMPONENTS: 

THE HEADSET 

The headset consists of three components: 
the earphones, the earphone sockets, and the 
supporting headband, neckband, or aviation 
helmet/ Its performance depends in part upon 
the functioning of each of the components sep¬ 
arately and in part upon the way in which they 
interact upon each other. 

The basic function of the earphone is to con¬ 
vert electric into acoustic power. Its effective¬ 
ness depends primarily upon the characteristics 
of the earphone itself, upon the dimensions and 

s For present purposes the aviation helmet is con¬ 
sidered a special type of headband. 






























136 


THE INTERPHONE 


volume of the enclosed space, headband pres¬ 
sure, and cushion leaks. The earphone socket 
serves the dual role of coupling the earphone 
to the ear and of excluding unwanted noise. 
Here again, the socket’s performance is modi¬ 
fied by secondary factors, such as the size and 
weight of the earphone and the headband pres¬ 
sure. From such considerations it is clear that 
the functioning of each component depends in 
part upon the combination that is used. 

The headset which provides a satisfactory 
speech-to-noise ratio must have good insulation 
and uniform frequency response. The headsets 
with which we began World War II were in¬ 
ferior on both counts. The Army Air Forces 



headset, HS-23 (see Figure 28), combined 
highly resonant earphones, R-14, with com¬ 
fortable, sponge rubber sockets which provided 
little insulation against frequencies below 
1,000 c. The Navy headset, NAF-38610 (see 
Figure 29), also used resonant earphones, 
CTE-49015 (TH-37), with poorly insulating 
sockets, CMO-49104 (TC-66), and a headband 
which did not supply sufficient pressure to seat 
the sockets properly. 

A series of studies on the necessary charac¬ 
teristics for headsets, 2 together with the active 
cooperation of leading manufacturers, led to 
the development of magnetic and dynamic ear¬ 
phones with uniform response up to 4,000 c. 
Earphone sockets designed by the Bell Tele¬ 
phone Laboratories and by the Psycho-Acoustic 


Laboratory provided greater insulation at all 
frequencies. Such improvements made it pos¬ 
sible for the AAF to adopt the magnetic ear¬ 
phone, ANB-H-1, and the Bell Laboratories’ 
socket, MX-41/AR, as components in their 
later headset HS-33 (see Figure 30). Although 
these sockets provided an excellent seal, they 
were not so comfortable as the Harvard Socket, 
M-301. The Harvard design, a circumaural type 
of “doughnut” socket, was optional for headset 
HS-33 but was not generally issued because it 
was feared that the lower sensitivity of the 
headset could not be made up in additional 
power. Furthermore, the chamois cushion did 
not withstand heat, dirt, and humidity well, a 



Figure 29. Headset NAF-38610. 


consideration in tropical and ground force ap¬ 
plications. The Harvard design was adopted, 
however, for use in aircraft helmets. 

The Navy adopted a dynamic earphone, 
ANB-H-1A, designed to meet the same specifi¬ 
cations as the Army’s magnetic ANB-H-1. This 
dynamic earphone, when combined with an 
improved headband and cushions of the dough¬ 
nut type, made up the H-4/AR headset (see 
Figure 31). 

Early in the war the Army Ground Forces 
adopted headset HS-30, shown in Figure 32. 
The response of the earphones R-30 fell off 
badly above 2,500 c, and the headband and tip 
were not entirely satisfactory. When properly 
adjusted, however, the semi-insert tip furnished 
good protection against ambient noise. The 






INTERPHONE COMPONENTS: THE HEADSET 


137 




Figure 32. Headset HS-30. 

discomfort of the HS-30, and the precarious 
seal provided by the H-16/U make these head¬ 
sets the least satisfactory of any types now in 
use. 

In general, circumaural sockets are more 


Figure 33. Headset H-16/U. 

fits closely around the ear and thus provides 
the advantage of comfort along with a rela¬ 
tively small enclosed volume (about 11 cu cm). 
This socket was adopted by Canada and is 
shown in Figure 34. 


semi-insert tip was retained, somewhat less 
effectively, in the later headset H-16/U (see 
Figure 33). The semi-insert coupler in this case 
is used in addition to a circumaural socket. The 
poor frequency response of the R-30 unit was 


Figure 30. Headset HS-33. 

retained, but the headband was greatly im¬ 
proved. The H-16/U has by far the most 
complicated construction of any of the head¬ 
sets considered herein. Altogether, the poor 
frequency response of the R-30, the serious 


comfortable, but they provide a larger volume 
and less efficient coupling between earphone 
and eardrum than supra-aural cushions. The 
volume driven by an earphone in the supra- 
aural MX-41/AR socket is about 5 to 6 cu cm; 


Figure 31. Headset H-4/AR. 

with the Harvard circumaural sockets it is 
about 22 cu cm. The effect of the larger volume 
is essentially a loss in sensitivity of 5 to 10 db, 
depending upon the type of earphone. A more 
recent circumaural socket of Harvard design 










138 


THE INTERPHONE 


Comfort is likewise an important factor in 
determining headband pressure. Lowering the 
pressure increases the comfort, but decreases 
the acoustic efficiency of the headset by allow¬ 
ing leakage around the socket. Leaks permit 
ambient noise to reach the listener’s ears, and 
also produce a low-frequency resonance be¬ 
tween 100 and 1,000 c. Data are available 12 
which indicate the pressure necessary for each 
type of socket. 

Articulation tests with dynamic (ANB-H- 
1A) and magnetic (ANB-H-1) earphones in 



Figure 34. Canadian headset ZA/CAN 1637 
with Harvard design 8-C earphone socket. 


three types of sockets 3 showed that the de¬ 
creased sensitivity of sockets enclosing a large 
volume is directly reflected in the articulation 
scores. If the volume is large, more gain is 
needed in the interphone amplifier. The same 
tests also demonstrated that the dynamic ear¬ 
phones are slightly superior to the magnetic, 
regardless of which earphone sockets are used. 


95 THE INTERPHONE AT ALTITUDE 

As our fighting planes went to higher and 
higher altitudes, new problems and new com¬ 
plications were added to the already difficult 
task of communication in high ambient noise. 
Frequency-response characteristics change, 
sensitivity decreases, and talkers find it ardu¬ 
ous or impossible to maintain an adequate voice 


level. Failure of interphone communication at 
altitude was a regular occurrence in the early 
days of high-altitude flying. 

The various ills which afflict the interphone 
at altitude can be conveniently summarized in 
the term altitude decrement [AD]. The altitude 
decrement of an interphone is the sum of the 
decrements of its components, including the 
effects exhibited by listener and talker. In one 
way or another the altitude decrement can be 
traced back to the effects of reduced ambient 
pressure. Temperature changes from 20 C at 
5,000 ft to — 20 C at 35,000 ft are also en¬ 
countered, 83 but tests have shown that changes 
due to temperature are of less importance to 
interphone performance than are pressure 
changes. Altitude decrement can thus be defined 
as the change in performance due to the reduc¬ 
tion of the ambient pressure. 

The effect of altitude on the human voice 
was described in Section 4.1.3 for the case 
where the talker does not wear an oxygen mask. 
Figure 14 in Chapter 4 shows the average 
altitude decrement for the three vowels u, a, 
and e, which is about 10 db between 400 and 
4,000 c. 

A more practical problem, however, is to 
determine the talker’s behavior when he is 
wearing an oxygen mask. Experiments were 
undertaken by the Electro-Acoustic Labora¬ 
tory 4 with a crew of trained men in an altitude 
chamber. These talkers spoke with “half effort” 
(see Section 4.1.3) at sea level and at altitude, 
and an audio-spectrometer was used to analyze 
the speech sounds. Several types of oxygen 
masks were used with a magnetic mask micro¬ 
phone (MC-253-A). The magnetic instrument 
avoids the complication of nonlinearity inher¬ 
ent in carbon microphones. The altitude decre¬ 
ment for the microphone (see Section 10.5) 
was then subtracted from the total decrement 
of voice-mask-microphone to obtain the decre¬ 
ment of the voice-mask combination. 

The results of this investigation showed that 
the altitude decrement of the voice-mask 
combination is largely independent of the type 
of oxygen mask. The decrement is different, 
however, for different speech sounds. As a 
general rule the average altitude decrement 
of a given voice-mask combination at altitude 






THE INTERPHONE AT ALTITUDE 


139 


A., relative to altitude A 1 is given approxi¬ 
mately by the expression, 

AD=10 1og air ^ enSi f 

air density at A» 

Figure 35 shows the average altitude decre¬ 
ment at 35,000 ft with a number of talkers 



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Figure 35. Average altitude decrement, experi¬ 
enced by talkers using oxygen masks, for various 
speech sounds at 35,000 ft. (The decrement of the 
microphone is not included.) 

for different speech materials. Comparison 
with a similar curve obtained without an oxy¬ 
gen mask (see Figure 14, Chapter 4) reveals 
that the effect of the mask is to boost the levels 
at low frequencies. Between 400 and 4,000 c, 



Figure 36. Peak instantaneous voltage at micro¬ 
phone terminals during articulation tests con¬ 
ducted in flight. (ANB-M-C1 microphone in A-14 
oxygen mask.) 

however, the presence of the mask increases 
the altitude decrement. 

It should be stressed that these data were 
obtained with the talkers maintaining the same 
subjective effort at altitude as at sea level. In 


actual practice the talker monitors his voice in 
his own earphones, and when his voice level 
drops at high altitudes he will exert additional 
effort to maintain an adequate level. Thus his 
altitude decrement will be somewhat smaller 
than indicated above. For engineering pur¬ 
poses, the “half-effort” decrement can be re¬ 
garded as including a desirable safety factor. 

The decrements which are actually observed 8b 
under flight conditions, where the talker moni¬ 
tors his voice level by his own side-tone, are 
indicated in Figure 36. These measurements, 
made in a B-17 bomber at Eglin Field, represent 
the decrement of the voice-mask-microphone 
combination as a function of altitude. Eleven 
announcers with ANB-M-C1 microphones in 
A-14 oxygen masks were used. The altitude 
decrement in this case amounts to about 6 db 
at 35,000 ft. 

The variability in speech level from word to 
word for a given talker is approximately the 
same at altitude as it is at sea level. Differences 
between talkers are slightly greater at altitude 
than at sea level. The range of signal levels 
which the interphone amplifier is required to 
handle is, therefore, about the same at all 
altitudes; it is only the mean speech level which 
changes. 

The altitude decrement of the voice-mask- 
microphone combination does not represent the 
total decrement for the interphone. The sensi¬ 
tivity of the earphones also changes at high 
altitudes. As the atmospheric pressure is re¬ 
duced, the compliance of the air volumes in¬ 
creases, but the impedance of the diaphragm 
remains the same. The effect of this change 
depends upon the design of the particular ear¬ 
phone considered, but in general the frequency 
response is altered and the sensitivity is de¬ 
creased (see Section 10.5). 

In order to evaluate the effect of these com¬ 
bined decrements upon articulation efficiency 
under actual conditions of flight, an extensive 
testing program was undertaken at Eglin Field, 
Florida, 5 ’ 0 - 8 with the cooperation of the Air¬ 
craft Radio Field Laboratory and of the Army 
Air Forces Proving Ground Command. A repre¬ 
sentative testing crew of AAF men was selected 
and trained. During flights in B-17F bombers, 
the test crew was stationed as indicated in 

















































140 


THE INTERPHONE 


Figure 37. Two experimenters participated in 
all the flights. 

Typical results from articulation tests con¬ 
ducted under these conditions are presented in 


Figure 38. h The data were obtained with the 
ANB-H-1 earphones and an experimental inter¬ 
phone amplifier which permitted adjustment 
of the voltage gain in the system. In Figure 38 



RAOIO COMPARTMENT 


FLIGHT ENGINEER 


NOSE 

THREE ARTICULATION CREWMEN 


PILOT'S 


PILOT, CO-PILOT 


Figure 37. Crew positions in B-17F. 



Figure 38. Functions showing the relation be¬ 
tween word articulation and voltage gain at 
5,000 and 35,000 ft. 


the articulation of the system is plotted as a 
function of the voltage gain of the interphone 
amplifier, with altitude as the parameter. From 
these functions it is apparent that, by boosting 
the amplifier gain at high altitude, about one- 
third to one-half of the altitude decrement can 
be recovered. 

On the basis of these and similar tests con¬ 
ducted both at Eglin Field and at the Psycho- 
Acoustic Laboratory, recommendations were 
made for interphone amplifiers (see Section 
9.3) to give the optimal performance at alti¬ 
tudes. It is obvious, of course, that the advan¬ 
tage from increasing the voltage gain of the 
amplifier is counteracted by overload distortion 
if the power output is limited. The tests showed 

11 The difference in performance between 5,000 and 
35,000 ft shown in Figure 38 is somewhat larger than 
the difference indicated for comparable equipment in 
Figure 1 of this chapter. The larger difference in 
Figure 38 resulted from the use of more difficult test 
words in this series of tests. 
















THE INTERPHONE AT ALTITUDE 


141 


that the peak power per head set should be at 
least 200 milliwatts, preferably 400 milliwatts. 
The improvements in articulation which result 
from increased gain and power constitute very 


real advantages to bomber personnel to whom 
the interphone represents the key to teamwork 
necessary for the successful completion of a 
mission. 



Chapter 10 

TEST METHODS AND EQUIPMENT FOR INTERPHONE COMPONENTS 


I N this chapter a list of test methods and 
equipment which are applicable to inter¬ 
phones and their components is given. This list 
covers all the types of tests which were found 
useful, with the exception of the articulation 
test discussed in Chapter 5. 

A considerable part of the activities of the 
Psycho-Acoustic and Electro-Acoustic Labora¬ 
tories during World War II was spent in assist¬ 
ing the Services (represented chiefly by the 
Army Air Forces and the Navy, Bureau of 
Aeronautics) in the improvement of aircraft 
communications as a whole, and in the procure¬ 
ment of suitable instruments. To accomplish 
this, the Harvard group functioned as an inde¬ 
pendent test laboratory which, at the request 
of the Services, assisted manufacturers of 
interphone equipment in the specific problems 
they had to meet. The results of various tests 
are contained in an extensive series of re¬ 
ports 9> n ’ 13 ' 15,17,18,20 " 24,25, 27,28,30 


10 1 METHODS AND EQUIPMENT FOR 
TESTING MICROPHONES USING 
HUMAN TALKERS 

The microphones in use in military communi¬ 
cations are, without exception, closely coupled 
acoustically to the talker, and the resulting 
interaction causes the performance of the 
microphone to vary according to the talker 
using it. For reasons of simplicity and speed, 
therefore, an artificial voice is often substi¬ 
tuted for the talker in laboratory tests (see 
Section 10.2). 

In many cases the substitution in laboratory 
tests of an artificial voice for the talker is per¬ 
missible, but in other instances (throat micro¬ 
phones, oxygen masks, noise shields, etc.) such 
a procedure is questionable and subject to large 
errors. Consequently, it is sometimes necessary 
to use the more elaborate procedures involving 
the use of real talkers wearing or using the 
equipment. 


10,1,1 Frequency Response 

The real-voice response (see Section 9.1.3) 
of a microphone can be considered a valid 
measure of the response to speech under repre¬ 
sentative conditions of use. Extensive tests of 
a variety of types of microphones were carried 
out at the Electro-Acoustic Laboratory, using 
a crew of three talkers and the audio-spec¬ 
trometer (see Section 4.1.1). The standard test 
sentence, “Joe took father’s shoebench out; 
she was waiting at my lawn,” devised by the 
Bell Telephone Laboratories, was used. 38 

The talkers were seated in a room free from 
acoustic wall reflections. The free-field sound 
pressure at a distance of 1 m in front of the 
talkers’ lips was measured by means of a 
condenser microphone (WE Type 640-AA) and 
sound-pressure meter (see Section 10.6). The 
output was analyzed by an audio-spectrometer 
in 12 contiguous frequency bands covering the 
range from 60 to 9,000 c. The long-time average 
value of the square of the microphone output 
voltage in each frequency band was measured 
simultaneously in the 12 channels. A thirteenth 
channel measured the overall value. 

The talker was then instructed to use the 
microphone under test. Its output voltage was 
analyzed by the audio-spectrometer. The ratio, 
in decibels, of the microphone output voltage 
in a band to the corresponding free-field sound 
pressure yields the real-voice response of the 
microphone (see Section 7.1.3). The talker 
was instructed to talk with what he judged 
was “constant effort” with and without the 
microphone under test. Certain vowels were 
used for this purpose. A standard volume indi¬ 
cator was used to adjust the overall average 
level of speech to a free-field sound-pressure 
level of about 74 db when talking into open air. 

If the response of a microphone so obtained 
is to be a valid measure of performance, cer¬ 
tain precautions in experimental technique and 
evaluation of the results are necessary. For 
example, the results obtained with carbon in- 


142 


TESTING MICROPHONES USING HUMAN TALKERS 


143 


struments have to be interpreted with caution, a 
since the results may depend on the level of 
speech, the spectrum of the speech signal, posi¬ 
tion of the instrument with respect to gravity, 
carbon current, and history of the carbon 
granules. Breath noises must be guarded 
against. The type of speech sounds used to test 
the microphone is important in all cases. Special 
attention must be given to this problem in the 
case of throat microphones where response to 
consonants may easily be simulated by response 
to harmonics of strong vowel sounds. For a 
more valid test, special speech material may 
have to be used. 



Figure 1. Real-voice response of a type T-17 
carbon microphone. 



Figure 2. Real-voice response of a Type T-38 
carbon microphone. 


Detailed data are available for about 40 
microphones tested at the Electro-Acoustic 
Laboratory. 38 Figures 1 through 8 show the 

a This is true with real or artificial voices. 


real-voice responses for several microphones. 
They include microphones mounted in a noise 
shield (Harvard Design D-18) and in an 
oxygen mask (A-14). Attention should be 



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Figure 3. Real-voice response of a type M-6/UR 
carbon noise-canceling microphone. 



Figure 4. Real-voice response of a type T-45 
carbon noise-canceling microphone. 



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Figure 5. Real-voice response of a type ANB- 
M-Cl carbon microphone in a Harvard type D-18 
noise shield. 























































































































































































144 


INTERPHONE COMPONENTS 



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Figure 6. Real-voice response of a type MC- 
253A magnetic microphone in a Harvard type 
D-18 noise shield. 


as a combination. This entails a drop in micro¬ 
phone output for frequencies below about 2,000 
c at a rate of about 6 db per octave when the 
microphone is measured as a separate unit. 

101,2 Insulation against Ambient Noise 

Tests on a number of microphones were per¬ 
formed at the Electro-Acoustic Laboratory to 
evaluate the noise picked up in a random sound 
field. 38 ® The talker (who was silent during the 
noise measurements) was seated in a room 
equipped with hard polycylindrical protrusions 



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Figure 7. Real-voice response of a type MC- 
253 magnetic microphone in a type A-14 oxygen 
mask (sea level). 



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Figure 8. Real-voice response of a type T-30-V 
carbon throat microphone. 

drawn to the fact that the response of the 
microphone element used in such enclosures 
must suitably complement the acoustic effects 
of the enclosure for acceptable performance 



Figure 9. Room with polycylindrical wall con¬ 
figurations for tests in random noise fields. 


(Figure 9). A random noise field of 85-db spec¬ 
trum level (approximately uniform with fre¬ 
quency) was set up, and the output voltage of 
the microphone due to noise was measured with 
an audio-spectrometer (see Section 9.1.4). For 
carbon instruments, the procedure was re¬ 
peated for a spectrum level of 70 db. Figures 
10 through 15 show the output voltage per cycle 
of six microphones in noise. 

Clearly, the speech-to-noise ratio of a micro¬ 
phone can be determined directly for a given 












































































































































TESTING MICROPHONES USING HUMAN TALKERS 


145 


speech level and noise spectrum by analyzing 
the output voltages due to speech and noise. 
By suitable choice of test conditions, mean¬ 
ingful comparisons between microphones are 


easily made (see Section 9.2). The exact charac¬ 
teristics of the analyzing device are of sec¬ 
ondary importance, since only voltage ratios 
are measured. Audio-spectrometers with both 



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Figure 10. Voltage generated by a type T-17 
carbon microphone used in a random noise field 
of uniform spectrum level. 


a: 

UJ 

a 


UJ 

o 


O 

> 


if) 

2 

ac 



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Figure 13. Voltage generated by a type T-45 
carbon noise-canceling microphone used in a 
random noise field of uniform spectrum level. 



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Figure 11. Voltage generated by a type T-38 
carbon microphone used in a random noise field 
of uniform spectrum level. 



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Figure 12. Voltage generated by a type M-6/UR 
carbon noise-canceling microphone used in a 
random noise field of uniform spectrum level. 


oe 

UJ 

CL 


UJ 

O 


if) 

2 

<r 



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Figure 14. Voltage generated by a type ANB- 
M-Cl carbon microphone in a Harvard type D-18 
noise shield used in a random noise field of 
uniform spectrum level. 



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Figure 15. Voltage generated by a type MC- 
253A magnetic microphone in a Harvard type 
D-18 noise shield used in a random noise field of 
uniform spectrum level. 





































































































































































































































































146 


INTERPHONE COMPONENTS 


linear and square-law characteristics have been 
used successfully for such measurements. 

It has been observed that if a microphone 
is very closely coupled to the mouth the speech- 
to-noise ratio is no longer independent of the 
movements and position of the mouth of the 
talker for high levels of ambient noise. The 
noise-canceling lip microphone (Type M-6/UR) 
provides a good example of this effect. A high¬ 
speed-level recorder was used to record the 
microphone voltage due to speech and the 
voltage due to noise as modified by the configu¬ 
ration of the talker’s mouth while he silently 
articulated the test sentence, “Joe took father’s 
shoebench out; she was waiting at my lawn,” 
with long pauses between the words. 34 It was 
found that the speech-to-noise ratio so obtained 
was, on the average, about 5 db lower than the 
“static” value in which the talker kept his 
mouth closed throughout the noise level meas¬ 
urement. 

These tests have a direct bearing on the use 
of voice-operated relays which have been sug¬ 
gested for use in very high ambient noise levels. 
Such relays would protect the interphone and 
crew from the constant noise picked up between 
messages. It has been found, 33 however, that the 
speech-to-noise ratio provided by the M-6/UR 
in a noise level prevailing in a F7F Navy air¬ 
plane (see Chapter 2) is not sufficient for 
reliable operation of such a relay. Some meas¬ 
ure of success was had by the substitution of 
an ANB-M-C1 microphone in an A-14 oxygen 
mask, provided care was taken to obtain a per¬ 
fect seal of the mask around the face and 
consequently more favorable noise exclusion. 

For tests of the type described above, and 
also for articulation tests in ambient noise, 
suitable electric noise generators have been 
developed at the Psycho-Acoustic and Electro- 
Acoustic Laboratories. Random noise is con¬ 
veniently produced by gaseous discharge 
tubes 10 or by using the thermal noise across 
the terminals of a resistor. 3815 To simulate the 
low-frequency components present in the spec¬ 
trum of noise in airplanes and other combat 
vehicles, sharp pulses are injected into the 
random noise. The generator producing these 
pulses is usually synchronized with the power¬ 
line frequency of 60 c. 


10.2 METHODS AND EQUIPMENT FOR 
TESTING MICROPHONES USING AN 
ARTIFICIAL VOICE 

For testing a large number of microphones 
in a production testing laboratory, for quickly 
observing the effect of a modification in the 
design, and for ready comparison of a number 
of similar units, simplified test methods which 
eliminate human talkers are required. The 
talker is replaced by a suitably designed sound 
source, or artificial voice, and the response of 
the microphone under test is expressed in terms 
of the output voltage developed for a given 
sound-pressure level. In some procedures, the 
sound pressure is specified as the free-field 
sound-pressure level 1 m directly in front of 
the voice. In others, the sound-pressure level 
at the microphone face (grid) is specified. The 
advantage of the methods using an artificial 
voice lies primarily in simplicity, speed, and 
versatility at the expense of validity. Clearly, 



Figure 16. Photograph of the artificial voice 
standardized by the Joint Radio Board [JRB]. 





TESTING MICROPHONES USING AN ARTIFICIAL VOICE 


147 




Figure 18. Drawing of the artificial voice (JRB) showing a type ANB-M-C1 microphone in position 
for response measurements. 





























































































































































































































148 


INTERPHONE COMPONENTS 


throat microphones cannot be tested by this 
method. The results obtained with microphones 
mounted in an enclosure or with instruments 
whose performance depends critically upon the 
shape and configuration of the talker’s mouth 
and head are, as a rule, not valid in terms of 
real-voice responses. Nevertheless, even in those 
cases, measurements with artificial voices do 
provide the designer with a laboratory tool of 
some merit. 


10 21 Frequency Response 

The Joint Radio Board of the Joint Aircraft 
Committee [JRB] standardized in 1943 the test 
procedure and equipment for measuring the 
frequency-response characteristic of micro¬ 
phones. This action was based largely on recom¬ 
mendations submitted by the Electro-Acoustic 
Laboratory. The procedure applies to micro¬ 
phones used in open air. 

The artificial voice (see Figure 16) consists 
of a modified dynamic loudspeaker (driving 
unit WE 555) and is driven by a suitable power 
amplifier and audio-frequency oscillator. To 
calibrate it, a baffle 3 in. in diameter is used 
to set up a constant sound-pressure level of 
115 db at its center by means of a condenser 
microphone (WE 640-A or 640-AA) and a 
sound-pressure meter (see Section 10.6). To 
simulate the microphone under test, a dummy 
microphone case is used. The distance of the 
baffle from the edge of the artificial voice is 
14 in. The apparatus with baffle and sound- 
pressure meter in place is shown in Figure 17. 
After calibration of the artificial voice the 
dummy microphone case with the condenser 
microphone is removed and the microphone 
under test is substituted. The output voltage 
of the unknown microphone is then determined 
as a function of frequency (see Figure 18). 

For carbon instruments, suitable condition¬ 
ing and agitating procedures are prescribed. 
They are designed to provide a uniform condi¬ 
tion of the carbon granules, and to provide 
also for a simulated agitation similar to the one 
due to the ambient noise in conditions of actual 
use. 

Engineering drawings and full details of the 


testing procedure are to be found in refer¬ 
ence 19a. 

Test apparatus of a somewhat more elaborate 
form, which permits testing of carbon instru¬ 
ments at various positions, is shown in Figure 
19, with a set of baffles and dummy cases. 



Figure 19. Artificial voice with various baffles 
and dummy microphone cases, suitable for deter¬ 
minations of the position effect in carbon instru¬ 
ments. 

To facilitate comparison with real-voice 
response characteristics, the results obtained 
by the JRB technique were expressed in terms 
of voltage developed per unit free-field sound 
pressure at a distance of 1 m from the artificial 
voice. This was roughly achieved by using the 
inverse square law to convert from a distance 
of 14 in. to 1 m. Clearly, this procedure is only 
very approximate; besides, it ignores the non¬ 
linearity of carbon instruments and the obstacle 
effects of the microphone housing.^ 

In Figures 20 through 23 a comparison is 
given between the real-voice response and the 
artificial-voice response of a T-17, T-38, 







TESTING MICROPHONES USING AN ARTIFICIAL VOICE 


149 


M-6/UR, and T-45 microphone. Differences of 
this type and magnitude justify, in many cases, 
the use of an artificial voice. Note that these 
microphones are designed for operation in open 
air. 



100 


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Figure 20. Showing a comparison between real- 
voice and artificial-voice response of a type T-17 
carbon microphone. 


number of instruments are given in report 
form. 19b 

Clearly, the artificial voice described is not 
suitable for testing oxygen masks and noise 
shields. For this purpose, an artificial voice was 
designed at the Electro-Acoustic Laboratory. 190 
It consists principally of a loudspeaker driving 
unit (WE 555) and a dummy head (see Figure 



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Figures 24 and 25 show artificial-voice cali¬ 
brations of ANB-M-C1 and MC-253 micro¬ 
phones tested in open air outside the enclosures 
for which they are designed. Note the rising 
characteristic at low and medium frequencies. 



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Figure 21. Showing a comparison between real- 
voice and artificial-voice response of a type T-38 
carbon microphone. 

This rise compensates for the enhanced re¬ 
sponse in that range when the microphones are 
used in an enclosure. Figures 26 and 27 show 
response characteristics of two carbon instru¬ 
ments taken at various levels of input sound- 
pressure level. Response curves in a large 


Figure 22. Showing a comparison between real- 
voice and artificial-voice response of a type 
M-6/UR carbon noise-canceling microphone. 

28). A tube of high acoustic impedance is 
brought through an opening in the head and 
terminated in the mouth of the dummy. The 
tube diameter (% in.) is comparable with the 
dimensions of the opening in the human mouth 



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Figure 23. Showing a comparison between real- 
voice and artificial-voice response of a type T-45 
carbon noise-canceling microphone. 

during speech. To obtain a high internal 
impedance, the brass tube is partially filled 
with an absorbing material (Fiberglas). The 
dummy head is made of plaster and covered 


























































































































































150 


INTERPHONE COMPONENTS 


with about (4 in. of Korogel. The shape of the 
head was one of a series designed for the 
Aero-Medical Laboratory at Wright Field, 
Dayton, Ohio, and represents the average 
measurements of a large number of individuals. 

Because of the high impedance of the artifi¬ 
cial voice, it is not feasible to generate signal 
levels approximating those found in use (115 



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Figure 24. Artificial-voice response (JRB) of a 
type ANB-M-Cl carbon microphone designed for 
use in oxygen masks. The microphone was meas¬ 
ured without the mask. 

to 120 db inside the mask). A magnetic mask 
microphone (MC-253-A) was used in the 
measurements instead of the usual carbon in¬ 
strument (ANB-M-Cl), since the linearity of 
the magnetic unit permitted extrapolation to 
the high signal levels. An artificial-voice cali- 


mate overall characteristic of the microphone- 
mask combination. 

Figure 29 shows a comparison between real- 
voice and artificial-voice calibration of an A-14 
oxygen mask with an MC-253-A microphone. 
The agreement is fair only at low and medium 
frequencies. Artificial-voice responses of 12 
masks have been compiled in report form. 19c 



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Figure 25. Artificial-voice response (JRB) of 
a type MC-253-A magnetic microphone designed 
for use in oxygen masks. The microphone was 
measured without the mask. 

10.2.2 Insulation against Ambient Noise 

In an effort to provide equipment for rapidly 
measuring the relative response of microphones 
to speech and to ambient noise, especially in 
the case of the noise-canceling (differential) 



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Figure 26. Artificial-voice response of a type 
ANB-M-Cl cai’bon microphone for various levels 
of acoustic input. 

bration of the ANB-M-Cl in open air by the 
JRB method (using 115-db signal level) and 
the MC-253-A was used to obtain the approxi- 


Figure 27. Artificial-voice response of a type 
T-17 carbon microphone for various levels of 
acoustic input. 

microphone, the Joint Radio Board of the Joint 
Aircraft Committee established standard pro¬ 
cedures in 1944. This equipment consists essen- 




































































































































































TESTING MICROPHONES USING AN ARTIFICIAL VOICE 


151 


tially of an artificial voice of the type described 
in Section 10.2.1 mounted in a square box lined 
with absorbing material. “Noise” is introduced 
from a conventional loudspeaker mounted in 
one of the walls (see Figure 30). The noise and 
speech spectra are simulated by line spectra 



Figure 28. Harvard artificial voice with dummy 
head. 


-20 


-30 



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Figure 29. Showing a comparison between real- 
voice and artificial-voice response of an MC-253-A 
microphone in an A-14 oxygen mask. 


consisting of 10 discrete frequency components 
suitably adjusted in relative amplitudes. The 
method and equipment are described in detail 
in a report published by the Electro-Acoustic 
Laboratory. 38b 

The method is primarily useful for produc¬ 
tion testing and to give the design engineer 
relative estimates of performance. For valid 
results, speech-to-noise ratio measurements 
with live talkers are necessary (see Section 
10 . 1 . 2 ). 



Figure 30. Photograph of artificial voice and 
noise box for testing noise-canceling lip micro¬ 
phones. 


Nonlinear Distortion 

To measure nonlinear distortion, a source 
of sound is necessary which generates high 
(120 to 130 db) sound pressures of sinusoidal 
wave form at the microphone. An artificial 
voice of the type already described does not 
fulfill these requirements. It was necessary, 
therefore, to design a special sound source. 

It can be shown that a combination of two 
rigid, coaxial tubes of different diameters and 
lengths will resonate at a succession of fre¬ 
quencies which are not harmonically related. 
If such a tube system is driven by a sound 
source (loudspeaker driving unit) at one of 
its resonant frequencies and the other end is 
terminated by the microphone under test, a 
large sinusoidal sound pressure is built up 
there. Harmonic components present at the 
driving unit will not be reinforced by the tube. 
With suitable tube dimensions, a frequency 
range from 200 to 4,500 c can be covered. 

























































152 


INTERPHONE COMPONENTS 


Operating frequencies are restricted to the 
resonant frequencies of the system. The output 
voltage of the microphone is analyzed with 
a suitable wave analyzer, and the harmonics 
can be expressed in the customary manner 
relative to the fundamental. Combination tone 
distortion can also be measured 1911 if two sinu¬ 
soidal input signals are used. 


10.3 METHODS and equipment for 
TESTING EARPHONES ON THE 
HUMAN EAR 

To determine the performance of an ear¬ 
phone on the human ear, some quantitative 
estimate must be made of how the ear responds 
to the sound from the earphone. Fundamentally, 
this requires measurements of the psycho¬ 
acoustic type. 


10-31 Frequency Response 

A listener wearing an earphone experiences 
a sensation of loudness when the earphone is 
energized electrically. If the loudness experi¬ 
enced by a typical listener is the measure of 
the response of an earphone, the listener’s 
judgment of, for instance, equal loudness is 
required in order to obtain a quantitative meas¬ 
urement. It will be recalled that a response 
measurement of this type was said to be the 
real-ear response (see Section 9.1.3). 

If, however, the sound pressure developed 
at a specified place under the earphone cushion 
is taken as a measure of the response, the ear 
is used only as an (passive) acoustic terminat¬ 
ing impedance for the earphone. While meas¬ 
urements of both types have been made, those 
involving loudness judgments are the more 
laborious and time consuming. 

Real-Ear Response 
(Loudness-Balance Technique) 

The real-ear response of an earphone with 
cushion and headband was determined in the 
following manner. The subject was placed in 
the sound field of a loudspeaker mounted in 
an anechoic chamber (see Section 10.9). The 


center of his head was located approximately 
on the axis of the loudspeaker, which he faced 
at a distance of several feet. The sound field 
near the subject’s head was essentially that of 
a plane wave. 

The sound-pressure level was adjusted to 84 
db at the position to be occupied by the subject 
for all test frequencies. The observer was then 
placed in the sound field. He wore a wide-range 
dynamic earphone (transfer standard) with an 
individually molded earpiece in his right ear. 
By means of an oscillator, an electronic switch, 
and two amplifier channels, sound was alter¬ 
nately generated by the loudspeaker and by 
the transfer standard. The subject was re¬ 
quired to adjust the voltage applied to the 
transfer standard until the two tones were 
equal in loudness. 

Now, the earphone and cushion under test 
were placed by the observer on his left ear with¬ 
out disturbing the transfer standard in his 
right ear. The voltages, determined in the 
previous step, were again applied to the trans¬ 
fer standard and the tone switched alternately 
to it and to the earphone under test. By means 
of another judgment of equal loudness, the 
voltage required to produce a loudness equal to 
that of a progressive plane wave of 84 db was 
determined. This is the real-ear response of the 
earphone and cushion under test. 

The procedure just outlined obviates the 
necessity of disturbing the earphone while 
making the judgment. Figure 31 shows 
the average real-ear response of a wide- 
range dynamic earphone (Permoflux PDR-10) 
mounted in a sponge rubber cushion (MX- 
41/AR) of the supra-aural type. Five ob¬ 
servers were used to arrive at the average 
Figure 32 shows the average real-ear response 
for a headset of the Navy sound-powered type, 
using a similar cushion (RCA MI-2045E). 

Probe-Tube Technique 

Measurements of the sound pressure actually 
developed under the earphone cushion have 
been carried out for earphones of ten different 
types with semi-insert tips and earphones 
using supra-aural cushions. 36 The sound pres¬ 
sure was measured near the entrance of the ear 
canal. 



TESTING EARPHONES ON THE HUMAN EAR 


153 


A thin, calibrated probe tube, suitably shaped 
and coupled to a Type 640-AA condenser 
microphone (similar to the instrument shown 
in Figure 10, Chapter 15) was attached to the 
earphone and cushion. The end of the probe 



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Figure 31. Real-ear response of a wide-range 
dynamic earphone (Permoflux PDR-10) mounted 
in a sponge-rubber cushion (MX-41/AR). (Con¬ 
stant applied voltage.) 

tube was pushed through the rubber walls of 
the cushion and was adjusted to lie in the plane 
through the face of the earphones and near its 
center. In the case of the semi-insert type ear- 



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Figure 33. Response of a wide-range dynamic 
earphone (Permoflux PDR-10) mounted in a 
sponge-rubber cushion (MX-41/AR). (Constant 
applied voltage.) 

phones, the end of the probe tube coincided 
with the plane through the end of the tip. By 
means of the probe tube the earphone response 
was determined with the earphone in place on 
the left ears of several subjects. 


Figures 33 and 34 show earphone response 
characteristics for a wide-range dynamic in¬ 
strument (Permoflux PDR-10), and a headset 
of the sound-powered type in use by the Navy 
during World War II. Figures 35 and 36 show 



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Figure 32. Real-ear response of a sound- 
powered earphone (type M). (Constant applied 
voltage.) 

the response of the HS-30 and H-16/U headsets 
used by the Army Ground Forces at the close 
of World War II. The bars shown in the figures 
give an indication of the standard deviation 



Figure 34. Response of a sound-powered ear¬ 
phone (type M). (Constant applied voltage.) 


of the observations for both frequency and 
level. 

Measurements of this type are useful in 
determining such important design features as 
quality of fit and seal and their variations 















































































































































































154 


INTERPHONE COMPONENTS 


among different subjects. The results also aid 
in the design of suitable artificial ears (coup¬ 
lers). (See Section 10.4.) 

It is interesting to make a comparison of the 



FREQUENCY IN CYCLES PER SECOND 

Figure 35. Response of a type HS-30 headset. 

(Constant applied voltage.) 

results obtained by the two methods mentioned 
for a given earphone and cushion. In the re¬ 
sponse measurement based on a loudness 
judgment, the ear is used in an active manner 
as an acoustic indicating instrument. In the 
response measurement based on the sound 
pressure under the cushion, the ear is acoustic¬ 
ally passive. To compare these two types of 
measurements, it is necessary to reduce both 
sets of data to some common reference, such 
as the sound pressure at the eardrum. To do 
this, it is necessary to have two further sets 
of data. First, in order to convert the real-ear 
response, the sound pressure generated at the 
eardrum by the sound source in front of the 
observer must be known for any given free-field 
pressure with the subject out of the field. To 
convert the second set of measurements, the 
ratio of the sound pressure at the eardrum to 
the sound pressure at the entrance of the ear 
canal is required. This ratio, combined with 
data of the type shown in Figures 33 through 
36, yields the sound pressure at the eardrum 
when the earphone is worn. 

The first conversion can be accomplished 
with the information in Figure 3 of Chapter 3. 
It was observed there that the pressure at the 
eardrum is greater than the free-field pressure, 
primarily because of the combined effects of 
diffraction around the head and pinna and 


resonance in the ear canal. By combining 
Figure 3 (see Section 3.1 for azimuth cp = 0) 
with Figure 31, Figure 37 is obtained. Figure 
37 shows the average sound pressure at the 



Figure 36. Response of a type H-16/U headset. 
(Constant applied voltage.) 


eardrum of a number of observers in a pro¬ 
gressive sound field which sounds equal in 
loudness to the tone generated by the earphone. 

Data for the second transformation have 
been obtained for a dynamic unit (Permoflux 



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Figure 37. Sound pressure at the eardrum 
generated by a progressive sound field. (Con¬ 
stant applied voltage.) 

PDR-10) mounted in an MX-41/AR cushion 
(see Figure 38) by using the flexible probe 
microphone described in Section 3.1. b By com¬ 
bining Figures 33 and 38, the sound pressure 
at the eardrum, as measured under the cushion, 
is obtained (see Figure 39). 

b The data are only an indication of the effects to be 
expected and are not generally valid, since only one 
subject was used. 



























































































































TESTING EARPHONES ON THE HUMAN EAR 


155 


The sound pressures so obtained (see Figures 
37 and 39) at the eardrum might be expected to 
be equal (within the experimental errors) 
since the two signals sounded equally loud to 


from a remote external sound source than when 
it comes from an earphone. Similar findings 
have been reported informally by the Bell Tele¬ 
phone Laboratories. Furthermore, the differ- 









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Figure 38. Ratio of the sound pressure at the 
eardrum to that at the cushion for a wide-range 
dynamic earphone. 


Figure 39. Sound pressure at the eardrum for 
constant applied voltage for a wide-range dy¬ 
namic earphone (Permoflux PDR-10). 


the average observer making the loudness 
judgment. Actually, the sound pressure gener¬ 
ated by the earphone appears to be larger than 
the one generated by the progressive sound 
field (see Figure 40). Similar results have been 

40 


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20 

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FREQUENCY IN CYCLES PER SECOND 

Figure 40. Showing the ratio of the sound pres¬ 
sure generated by the earphone to the sound 
pressure generated there by a progressive sound 
field sounding equally loud. 


























































































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obtained for another type of earphone, using 
a supra-aural cushion, but indications are 
strong that in this case the results depended 
to a large degree on the choice of observers. 

The disagreement in these results can be 
interpreted to mean that the ear appraises 
loudness differently when the signal comes 


ence reported 2 at low frequencies between the 
minimum audible pressure and minimum 
audible free field (see Section 3.2) appears to 
be a similar effect and one of the same magni¬ 
tude. 

Measurements on a number of earphones 
using the insert tips of the type used with hear¬ 
ing aids failed to show the difference dis¬ 
cussed above at frequencies below 1,000 c. 29 It 
must be concluded that the data described here 
are as yet insufficient to permit general con¬ 
clusions or explanations, and further research 
seems indicated. 

Threshold Technique 

There is still a third method of measuring 
the response of an earphone on the ear by 
utilizing the auditory threshold. If the threshold 
for the observer is known in terms of, say, 
sound-pressure level at the eardrum, a calibra¬ 
tion of the earphone can be arrived at. 
Measurements of this type have been carried 
out at the Psycho-Acoustic Laboratory 31 - 35 for 
many types of headsets, using a group of young 
male subjects. The threshold values reported 2 
for the minimum audible pressure were 
assumed to represent the average threshold 
of the subjects. Figures 41 and 42 show, for 
two types of headphones using a supra-aural 
















































































































156 


INTERPHONE COMPONENTS 


seal, a comparison between response curves 
obtained by the threshold method and by the 
probe-tube method. This comparison suggests 
the possibility that a uniform shift in the 



FREQUENCY IN CYCLES PER SECOND 

Figure 41. Showing a comparison between the 
response of a wide-range dynamic earphone ob¬ 
tained by the threshold and probe-tube methods. 

assumed threshold would result in better agree¬ 
ment. However, this is not supported by similar 
comparisons for several other earphones of 
various types. 


10 ' 3 ' 2 Insulation against Ambient Noise 

The presence of the earphone and earphone 
socket shields the ear acoustically from the 
effects of ambient noise. It is of great im¬ 
portance to measure the insulation (attenua¬ 
tion) provided by various types of earphones 
and cushions, since the resultant level of noise 
at the listener’s ear is a controlling variable in 
the performance of an interphone. 

Attempts have been made at the Electro- 
Acoustic Laboratory to measure the insulation 
by physical means, using a dummy head and 
replacing the eardrum by a condenser micro¬ 
phone. 380 Using a slightly different technique, 
the earphone itself was used as a detector for 
the sound pressure developed by the noise near 
the entrance of the listener’s ear canal. A con¬ 
denser microphone connected to a preamplifier 
by a flexible shielded cable was also used. Meas¬ 
urements were made with single frequencies in 
an anechoic chamber and with bands of noise 
in a room with hard walls. The results of these 


measurements show the necessity, at the pres¬ 
ent state of the art, of using humans to achieve 
valid conditions of fit and seal. It is indicated 
that measurements of the psychophysical type, 



FREQUENCY IN CYCLES PER SECOND 

Figure 42. Showing a comparison between the 
response of a sound-powered earphone (type M) 
obtained by the threshold and probe-tube methods. 

while more time-consuming and laborious, are 
to be preferred wherever possible. 

Two methods employing human observers 
have been used at the Psycho-Acoustic Labora¬ 
tory. One method of measuring the insulation 
provided by an earphone is based on a direct 
determination of the masked threshold of the 
listener wearing the headset under test and 
immersed in the noise field which corresponds 
to the conditions of use. 12 Single-frequency 
tones are used to energize the earphone, and the 
level is adjusted to be just audible in the 
presence of noise. From the real-ear calibration 
of the earphone, the equivalent free-held level 
of the tones can be found. The equivalent free- 
held spectrum level of the noise may be assumed 
to be K db below that value, where K (see 
Section 3.4.1) is the width of the critical band 
at that frequency. This spectrum represents the 
contribution to the total noise at the listener’s 
ear due to earphone leakage. 

Simpler, more hexible, and better suited for 
comparisons between earphones of different 
types is the method of measuring insulation 
at threshold in quiet. The threshold method 
involves the determination of the threshold of 
hearing at each of 14 frequencies in two ex¬ 
perimental situations: first, when the ear 
exposed to the testing tones is unimpeded, and 
























































































TESTING EARPHONES USING AN ARTIFICIAL EAR 


157 


second, when it is covered by the headset under 
test. The difference in decibels between the two 
thresholds is the amount of protection afforded. 

The measurements are made monaurally in 
an anechoic room, the observer seated with his 
right ear to a loudspeaker 2 m away. The left 



Figure 43. Insulation measurements for head¬ 
sets by the threshold method. 


ear is blocked off with an earplug and covered 
with an earphone socket. The observer sets an 
oscillator to the test frequency. This tone passes 
through an electronic switch which interrupts 
it at irregular intervals and is then amplified 
and impressed upon the loudspeaker. By the 
use of a 2-db-per-step attenuator, the observer 






















































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FREQUENCY IN CYCLES PER SECOND 


Figure 44. Insulation measurements for head¬ 
sets by the threshold method. 


adjusts the intensity of the sound until his 
threshold is reached. 

The observers should be selected for average 
or better-than-average hearing, and should 
have considerable experience in determining 


thresholds with different kinds of headsets 
before any of their results are used. Experience 
has indicated that reliable estimates can be 
obtained by averaging the thresholds for 8 
to 10 different observers. 

Insulation measurements made by the 
threshold technique are shown in Figures 43 
through 45. 35 Figure 43 shows the insulation 



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1000 10,000j 

FREQUENCY IN CYCLES PER SECOND 

Figure 45. Insulation measurements for head¬ 
sets by the threshold method. 

provided by the early AAF headset HS-23 and 
by the improved AAF headset HS-33. Figure 
44 shows the insulation provided by the Ground 
Forces’ headsets. Figure 45 compares the 
earlier (TH-37) and later (H-4/AR) U. S. 
Navy headsets. In all cases the adoption of 
improved cushions increased the amount of 
insulation provided (see Section 9.4). It will 
be noted that in all cases the noise exclusion 
is more effective for frequencies above than 
below 1,000 c. 


50 — 
100 


10.4 METHODS AND EQUIPMENT FOR 
TESTING EARPHONES USING AN 
ARTIFICIAL EAR (COUPLER) 

A large amount of work has been done in an 
effort to replace the human ear in response 
measurements on earphones by a physical 
model of simple geometrical shape and con¬ 
struction. Tests on such artificial ears (coup¬ 
lers) are far more suitable for the average 
testing laboratory, especially if production 
testing is involved. Inherently they are simpler 
at the expense of validity, as is usual in such 
situations. 

Two couplers have been designed, in coopera- 




































































































158 


INTERPHONE COMPONENTS 


tion with the Bell Telephone Laboratories, of 
volumes of 6 cu cm and 2 cu cm, respectively. 
They are designed to duplicate, as far as is 
consistent with simplicity of design, the per¬ 
formance of earphones using cushions with a 
supra-aural seal and semi-insert tips (see Sec¬ 
tion 11.1). A more complicated design for an 
artificial ear has been described in reference 1. 
Both devices are designed for use with the 


In 1942, the Joint Radio Board also adopted 
the design of a coupler of 2 cu cm volume 
similar to the one shown in Figure 5 in Chapter 
15. Subsequent information has shown that 
this design is suitable only for earphones of 
the hearing-aid type using insert tips. 

To investigate the extent to which these two 
couplers, shown in Figures 46 through 48, 
duplicate the performance of a given earphone 



Figure 46. Engineering drawings of the standard coupler of 6 cu cm volume for use with the Western 
Electric type 640-AA microphone with protective grid. 


WE 640-AA or 640-A condenser microphone 
as the pressure-indicating device. The larger 
coupler was standardized by the Joint Radio 
Board of the Joint Aircraft Committee in 1942. 
This coupler consists essentially of a cavity of 
simple geometrical shape, with means of 
coupling to it an ANB-H-1 or ANB-H-1A ear¬ 
phone and a condenser microphone. Engineer¬ 
ing drawings of the coupler of 6 cu cm volume 
to be used with Type 640-AA microphones with 
and without the protective grid are given in 
Figures 46 and 47. (A drawing of the coupler 
of 2 cu cm volume for earphones using semi¬ 
insert tips is shown in Figure 48.) 


on the human ear, measurements of the sound 
pressure developed under the cushion on the 
ear (see Section 10.3.1) can be compared with 
measurements on couplers. Such a study has 
been carried out for several samples of each 
of 10 different types of earphones. 30 

Figures 49 through 52 show such compari¬ 
sons for 4 different types of earphones. From 
these and other data, the following conclusions 
can be drawn. 0 


c It should be kept in mind that the sound pressure at 
the eardrum will be significantly higher than the pres¬ 
sure at the entrance of the ear canal at frequencies 
above 1,000 c. 




































































































































TESTING EARPHONES USING AN ARTIFICIAL EAR 


159 


1. The agreement between the average re¬ 
sponse in the ear and in the appropriate coupler 
is fair. If the earphone exhibits a resonant peak 
in the frequency range between 1,000 and 2,000 
c on the coupler, this peak is much reduced 
in height or not present at all when the ear¬ 
phone is loaded by the human ear. Incorpora¬ 
tion of suitable damping elements in the 
coupler should result in better agreement at 


tools for many tests on earphones using a supra- 
aural seal or a semi-insert tip. The performance 
of cushions of different types, such as the 
circumaural socket (see Section 11.1) which 
encloses a comparatively large volume, cannot 
be duplicated by these designs. Wave motion 
inside the cavity complicates the design of a 
coupler having such a large volume. The per¬ 
formance of circumaural cushions is best 



Figure 47. Engineering drawings of the standard coupler of 6 cu cm volume for use with the Western 
Electric type 640-AA microphone without protective grid. 


the peak, but this will be achieved at the 
expense of simplicity and duplicability. 

2. The upper cutoff region of the earphone 
response is reasonably well duplicated by the 
measurements on the couplers. 

3. Leakage effects, resulting in reduced 
levels at low frequencies and sometimes in 
elevated levels at medium frequencies when 
tested on the ear, were intentionally avoided in 
the coupler design for reasons of simplicity. 

It can be concluded that these two designs 
of artificial ears constitute useful and workable 


studied by the procedures using human ears, 
discussed in Section 10.3.1. 

It has been assumed, thus far, that the 
response of an earphone is measured with a 
constant voltage across its terminals. It is fre¬ 
quently desired to predict the performance of a 
given earphone energized from an amplifier 
with a significant output impedance. The im¬ 
pedance of a headset is, in general, complex and 
a function of frequency. The concept of constant 
power available, developed by the Bell Tele¬ 
phone Laboratories, is useful in predicting the 













































































































































160 


INTERPHONE COMPONENTS 



Figure 48. Coupler of 2 cu cm volume for use 
with earphones using semi-insert tips. 



FREQUENCY IN CYCLES PER SECOND 

Figure 51. Showing a comparison between the 
response obtained on the human ear by the probe- 
tube method and the response obtained on a 
suitable coupler (HS-30 headset). 



100 1000 10,000 
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Figure 49. Showing a comparison between the 
response obtained on the human ear by the probe- 
tube method and the response obtained on a suit¬ 
able coupler (Permoflux PDR-10). 


Figure 52. Showing a comparison between the 
response obtained on the human ear by the probe- 
tube method and the response obtained on a 
suitable coupler (H-16/U headset). 



Figure 50. Showing a comparison between the 
response obtained on the human ear by the 
probe-tube method and the response obtained on 
a suitable coupler (sound-powered earphone 
type M). 


Power Available » ^ 

Figure 53. Schematic illustrating the concept 
of available power. 














































































































































































































TESTING EARPHONES USING AN ARTIFICIAL EAR 


161 


performance and power requirements for head¬ 
sets when used with sources having a constant 
output impedance. Consider Figure 53. The 
amplifier of internal resistance R will deliver 
maximum power into a resistive load also of 
magnitude R. In this case the condition of 
“matched impedance” obtains. The power de¬ 
livered to the load resistor R is given by E-/AR, 
where E is the open-circuit voltage of the 
amplifier output stage. To deliver 1 mw to 
the load resistor R, a voltage E = V0.004.R v 
is necessary, where E is measured as shown in 
Figure 54. If a constant open-circuit voltage of 



2 3 4 5 6 7 8 9 2 3 456789 

10 100 1000 


R IN OHMS 

Figure 54. Showing the open-circuit voltage E 
as a function of the resistive source impedance R 
for 1 mw constant power available. 

that value is maintained, independent of fre¬ 
quency, and the headset (earphone) is substi¬ 
tuted for the load resistor R, the response 
measured under such conditions is said to be 
obtained for 1 mw constant power available. 
A set of response characteristics using different 
values of R is valuable in that it permits the 
selection of that value of R which gives the most 
desirable characteristic. Conversely, the re¬ 
sponse of a given headset driven from an 
amplifier of given internal resistance can be 
predicted. Measurement of the ratio E'/E (see 
Figure 53, bottom) permits the calculation of 
the response characteristic for constant applied 
voltage. Figures 55 through 58 show the fre¬ 
quency-response characteristics of a number of 
earphones for 1 mw constant power available, 
measured on a coupler. Note the significant im¬ 
provement in the performance of headsets used 


at the start of World War II (see Figures 55 
and 56) and at its close (see Figures 57 and 58). 

Couplers can be conveniently used to deter¬ 
mine nonlinear distortion of earphones. The 
sound-pressure meter (see Section 10.6) is 
suitable for this type of measurement, since it 



100 1000 10000 
FREOUENCY IN CYCLES PER SECOND 

Figure 55. Coupler response of an HS-23 head¬ 
set for 1 mw constant power available. 



Figure 56. Coupler response of a TH-37 head¬ 
set for 1 mw constant power available. 



Figure 57. Coupler response of an ANB-H-1 
earphone for 1 mw constant power available. 















































































































































































162 


INTERPHONE COMPONENTS 


is linear up to very high sound-pressure levels. 

A large body of physical data on a variety 
of earphones has been compiled in report 
form. 19 



Figure 58. Coupler response of an ANB-H-1A 
earphone for 1 mw constant power available. 


1°.5 METHODS for testing inter¬ 
phone COMPONENTS AT REDUCED 
AMBIENT PRESSURES AND 
TEMPERATURES 


rotation of the microphone about its axis and 
adjustment of its position relative to the direc¬ 
tion of gravity from the outside of the tank 
(see Figure 60). 

Most tests have been made at reduced ambi- 



Figure 59. Tank for testing interphone equip¬ 
ment at reduced ambient pressures and tempera¬ 
tures. 


Tests on microphones in oxygen masks using 
human talkers at altitude have been reported in 
Section 9.5. Tests using an artificial voice of 
the type described in Section 10.2.1 were also 
carried out, but their validity is extremely 
doubtful, since the artificial voice fails to react 
in certain important respects as does the live 
voice. The altitude decrement of microphones 
proper can, however, be determined with some 
confidence by artificial-voice methods. 

Direct determinations of the real-ear re¬ 
sponse at altitude would be difficult. It is 
customary to express the performance of ear¬ 
phones at altitude by determining the altitude 
decrement on a suitable artificial ear and 
assuming that the same decrement would hold 
on the human ear. 

To determine the response of an earphone 
or microphone at altitude the equipment is 
placed in a tank in which the desired ambient 
pressure and temperature are maintained (see 
Figure 59). For measurements on microphones 
an artificial voice is used similar to the one 
shown in Figure 19. To facilitate the test pro¬ 
cedure, electric motors are provided to permit 


ent pressure only, since this effect, in general, 
outweighs the temperature effect. The proce¬ 
dures necessary to secure absolute calibrations 
of the condenser microphone (which forms an 



Figure 60. Artificial voice with motor controls 
for use in the tank shown in Figure 59. 


integral part of the test equipment) at reduced 
ambient pressure and temperature is discussed 
in Section 10.7. 





























































THE SOUND-PRESSURE METER 


163 


Figures 61 and 62 show typical response 
characteristics for the carbon and magnetic 
mask microphones ANB-M-C1 and MC-253-A. 


10.6 the sound-pressure meter 

The sound-pressure meter, 16 incorporating 
the WE Type 640-AA condenser microphone, 
was designed at the Electro-Acoustic Labora- 


are feasible except in extremely humid weather. 
The voltage read on the vacuum-tube voltmeter 
is directly proportional to the open-circuit volt¬ 
age of the microphone and hence is proportional 
to the sound pressure. 

The amplifier (sometimes referred to as pre¬ 
amplifier) is housed in a cylindrical metal case 
11/2 in. in diameter and about 12 in. in length. 
The case is equipped with a thread and contact 
pin to accommodate the condenser microphone. 



o 


rr 









■—— SEA LEVEL 

- 20,000 FEET 

----- 40,000 FEET 













V 














rti 

v 








// / y 


\ 

\\ 











T 

\\ 






} 

tf 










s 










S' 

V 


















“TT 
























100 1000 10,000 

FREQUENCY IN CYCLES PER SECOND 



Figure 61. Artificial-voice response of a type 
ANB-M-C1 carbon microphone for various alti¬ 
tudes. 


Figure 62. Artificial-voice response of a type 
MC-253-A magnetic microphone for various alti¬ 
tudes. 


tory and adopted as the standard instrument 
for response measurements on microphones and 
earphones. Numerous specific applications have 
been mentioned already. 

The instrument consists essentially of a one- 
stage amplifier whose gain is approximately 
unity. Its main function is to act as an im¬ 
pedance tranforming device with provision for 
the necessary bias voltage of 200 v for the 
microphone. A suitable vacuum-tube, voltmeter 
is connected to the low-impedance output ter¬ 
minals. The input impedance is of the order of 
300 megohms. The frequency response is uni¬ 
form to within ±0.1 db between 100 and 
10,000 c. This is achieved by using a highly 
degenerative vacuum-tube circuit (cathode fol¬ 
lower) . It should be noted that the high input 
impedance is achieved despite the fact that the 
input resistance for low-frequency fluctuations 
is only 20 megohms. This substantially de¬ 
creases hazards due to possible leakage between 
the terminals of the condenser microphone. The 
overall noise level is such that measurements 
of sound-pressure levels above about 80 to 85 db 


Figure 63 shows the preamplifier with vacuum- 
tube voltmeter. The operating voltages for the 
instrument are supplied by a 6-v storage bat¬ 
tery and a 300-v B battery, or regulated power 
supply. With proper precautions, the storage 
battery can be replaced by a filament trans- 



Figure 63. Sound-pressure meter with vacuum- 
tube voltmeter. 





















































































164 


INTERPHONE COMPONENTS 


former. The sound-pressure meter, as commer¬ 
cially manufactured, includes, besides the pre¬ 
amplifier, a shielded battery box and a suitable 
control box containing the necessary meters. 
Full details covering operation, calibration, and 
servicing are available in report form. 16a 

Certain objections have been raised regard¬ 
ing the sound-pressure meter as described 
above, and a laboratory model of an improved 
amplifier has been designed. 19e It combines the 
best features of the earlier design with de¬ 
creased noise level, a-c operation on a-c power, 
reduced size, and potentially higher input im¬ 
pedance. A flexible extension cable of consid¬ 
erable length 19f connecting microphone and pre¬ 
amplifier can be used. This has advantages in 
cases where the weight of amplifier and micro¬ 
phone combined is objectionable (see Section 
10.3.2). A discussion of the effect of the ampli¬ 
fier input impedance on the noise level of a 
condenser microphone and ways of further 
improving the performance are given else¬ 
where. 195 


10 7 ABSOLUTE PRESSURE CALIBRATION 
OF CONDENSER MICROPHONE BY 
THE RECIPROCITY METHOD 

In the early years of World War II, there 
arose a great need for standardization of test 
equipment and procedures used by laboratories 
testing military interphone equipment. Accu¬ 
rate absolute pressure calibrations of the stand¬ 
ard microphones were necessary, especially 
since in many cases the performance of the 
instruments under test was closely specified. 

In most cases, pressure calibrations rather 
than free-field calibrations were needed. The 
first step, therefore, was to attempt the stand¬ 
ardization of methods for calibrating condenser 
microphones in absolute terms for use as sec¬ 
ondary standards. The condenser microphone 
most extensively used in this country for 
acoustic measurements is the Western Electric 
Type 640-AA microphone. Consequently, all 
work in absolute calibration of microphones at 
the Electro-Acoustic Laboratory was concerned 
with obtaining accurate pressure calibrations of 
this instrument. The predecessor of the Type 


640-AA microphone, the Type 640-A, possesses 
less favorable stability and temperature char¬ 
acteristics. The diaphragms of these older in¬ 
struments are made of Dural instead of steel, 
and are usually coated with blue lacquer for 
identification. Figure 64 shows a photograph of 
the 640-AA microphone. 


i j > ! 1 I 1 | 1 I 1 I 1 1 1 , 1 | 


el 1 j— * — 5 

I »* 


Figuke 64. Photograph of a Western Electric 
type 640-AA condenser microphone. 


Of the known methods for absolute calibra¬ 
tion of microphones in terms of sound pressure 
uniformly actuating the diaphragm, the reci¬ 
procity method has occupied a preferred place 
in recent years. This method combines accuracy 
and reliability with simplicity to a degree not 
equaled by other methods. Microphones with 
high diaphragm impedance, such as the con¬ 
denser type, are particularly well suited for 
calibration by the reciprocity method. 

The theory underlying the reciprocity method 
has been described in the literature. 5 - 6 The 
method rests on the application of the reci¬ 
procity principle to a four-terminal network 
formed by two condenser microphones coupled 
acoustically by a suitable cavity. 

An intensive research program aimed at per¬ 
fecting and standardizing the experimental 
procedures was carried on at the Electro- 
Acoustic Laboratory with the active coopera¬ 
tion of the National Bureau of Standards and 
the Bell Telephone Laboratories. As a result 
of these activities, it is now possible to obtain 
absolute calibrations of WE Type 640-AA mi¬ 
crophones accurate to ±0.1 db in the frequency 
range of from 50 to 5,000 c and to ±0.2 db to 










CALIBRATION BY RECIPROCITY METHOD 


165 


12,000 c. These results were confirmed by suc¬ 
cessive calibrations of a number of instruments 
at the three laboratories. Figure 65 shows the 
particular apparatus used at the Electro- 


Acoustic Laboratory. Figure 66 shows the 
coupling cavity for the two microphones with 
preamplifier assembly. (Full details are re¬ 
ported elsewhere.) 37 Figure 67 shows a repre- 



Figure 65. Equipment for absolute pressure calibration of condenser microphones by the reciprocity 
method. 



Figure 66. Coupling cavity and preamplifier for calibrating condenser microphones by the reciprocity 
method. 



































166 


INTERPHONE COMPONENTS 


sentative calibration curve for a Type 640-AA 
microphone. 

The frequency range of these measurements 
has been extended to about 20,000 c. Further¬ 
more, absolute calibrations were carried out 
at reduced ambient pressures and temperatures. 



100 1000 10,000 20,000 
FREQUENCY IN CYCLES PER SECOND 


Figure 67. Response of a typical Western Elec¬ 
tric type 640-AA condenser microphone. 



Figure 68. Equipment for absolute pressure 
calibration of condenser microphones by the 
reciprocity method at reduced ambient pressure 
and temperature. 



Figure 69. Response of a Western Electric type 
640-AA condenser microphone at various alti¬ 
tudes. 


For this purpose, the coupling cavity with the 
two microphones is placed in a tank where the 
required conditions of ambient pressure and 
temperature are maintained (see Figure 68). 
Figures 69 and 70 show two typical calibrations 
of Type 640-AA microphone at altitude and at 
low temperatures. No measurements were car¬ 
ried out showing the combined effect of low 
ambient pressure and temperature. 


ms METHODS FOR FREE-FIELD 
CALIBRATION OF MICROPHONES 

As distinguished from the pressure calibra¬ 
tion, the free-field calibration of a microphone 
is customarily defined as the ratio of its open- 
circuit voltage to the sound pressure existing 
at the location of the microphone prior to the 
insertion of the microphone into the sound 
field. The sound pressure is produced by a plane 
progressive sound wave in which the micro¬ 
phone is oriented at some specified angle with 
respect to the wave front. The ratio of these 
two calibrations is commonly referred to as the 
free-field correction. 

Free-field calibrations of microphones are 
required in many practical cases, notably in 
noise-level measurements (see also Chapter 2). 

It is customary, in such cases, to calibrate the 
microphone in terms of “random” incidence. 4 
This calibration is defined in terms of the free- 
field sound pressure measured by a “point” 
microphone in a diffuse, or random, sound field. 
The random incidence calibration can be com- 



Figure 70. Response of a Western Electric type 
640-AA condenser microphone at various ambient 
temperatures. 































































































































































THE ANECHOIC CHAMBER 


167 


puted approximately from free-field calibra¬ 
tions obtained in a plane wave for various 
angles of incidence. 20 It becomes of importance, 
therefore, to determine the plane-wave free- 
field calibrations of a number of various micro¬ 
phones for various angles of incidence. These 
calibrations were obtained by the substitution 
method, using the Type 640-AA condenser mi¬ 
crophone as the standard instrument. The free- 
field calibration of the standard was obtained 
by adding the free-field correction to the abso¬ 
lute pressure calibration obtained by the reci¬ 
procity method. This correction, shown approx¬ 
imately in Figure 71 for angles of incidence 


-1-—— 

"T" 






- 1 - 1 — 1 — 




































S’ 










/ 






INCIDENCE PERPENDICULAR f 
TO PLANE OF DIAPHRAGM K 

V 

7* 











— 





-- 









L 


L PARA 
DlAPt- 

IRAGM 











100 


3 4 9 6 7 8 9 

1000 


4 3 4 7 8 9 

10,000 


FREQUENCY IN CYCLES PER SECOND 


Figure 71. Approximate free-field corrections 
for Western Electric type 640-AA condenser 
microphones. 


normal and parallel to the diaphragm, was 
obtained from model experiments and from 
computations of diffraction and cavity-reso¬ 
nance effects. For more accurate work, a direct 
determination by the reciprocity method would 
have been preferable. 

The free-field correction for random incidence 
for a 640-AA microphone was also determined 
and is given elsewhere. 1911 All measurements 
were carried out in the large anechoic chamber 
described in Section 10.9. 


10.9 the anechoic chamber 

The need for an environment simulating the 
acoustic properties of unlimited expanse of still 
air has been apparent in many instances in the 
course of the preceding discussion. It is hard 
to underestimate the value of an anechoic cham¬ 


ber, i.e., a room free from reflections, as a 
research tool in the acoustic laboratory. In 
order to be effective, the dimensions of the 
room must be large and the walls must be lined 
with a sound-absorbing structure which pro¬ 
vides maximum and uniform sound absorption 
over the frequency range in which measure¬ 
ments are to be performed. For accurate meas¬ 
urements, temperature and humidity inside the 
chamber should be controlled. 

A large anechoic chamber was built to meet 
these needs at the Electro-Acoustic Laboratory 
in 1943. The design and construction of this 
chamber have been described in an earlier 
report. 32 

A photograph of the interior of the chamber 
is shown in Figure 72. The acoustic treatment 



Figure 72. Photograph of the interior of the 
large anechoic chamber at Harvard. 


of the walls is of a novel design, consisting of 
wedges of sound-absorbing material mounted 
perpendicular to the walls. This treatment 
has important advantages over treatments of 
older types 3 because it provides a structure of 
large depth which absorbs the low-frequency 
sounds efficiently and at the same time provides 
for a gradual transition from the acoustic im¬ 
pedance of air to the acoustic impedance of 




































































168 


INTERPHONE COMPONENTS 


P.F.-Fiberglas/ 1 the material used in the treat¬ 
ment. 

The general geometrical shape of the absorb¬ 
ing structure was suggested by a paper 7 de¬ 
scribing a room whose treatment consisted of 
muslin bags of pyramidal shape stuffed with 
loose rock wool (density about 12 lb per cu ft) 
and mounted side by side perpendicular to the 
walls. 

This general idea was used as the starting 
point at Harvard for the design of a better 
structure. About 500 different designs were in¬ 
vestigated experimentally. A structure was fi¬ 
nally selected whose main element is a wedge 
made of P.F.-Fiberglas and contained in a 
muslin bag. The wedge terminates in a rec¬ 
tangular base section. Butted side by side, this 
results in the formation of a solid blanket of 
P.F.-Fiberglas behind the wedges. The bases of 
the wedges are backed by two layers of Fiber- 
glas blanket, which in turn are separated by 
an air space from the solid concrete wall form¬ 
ing the chamber. A diagram of the total struc¬ 
ture is shown in Figure 73. 



Figure 73. Design data and dimensions for the 
wedge structure used to line the walls of the 
large chamber. 


In order to measure the sound-absorbing 
characteristics of the structures, the normal 
absorption coefficient a was measured for one 
structural unit by examining the standing wave 
pattern set up by a sound source in a tube, the 
termination of which was formed by the ele- 

d P.F.-Fiberglas, impregnated with phenol formalde¬ 
hyde, is a product of the Owens-Corning Fiberglas 
Company. 


ment under test (see Section 2.2.3). Instead of 
using a, the pressure-reflection coefficient can 
be used to describe the behavior of the struc¬ 
ture. It should be kept in mind that measure¬ 
ments of the type discussed above can, at best, 
yield only a basis for comparison of different 
structures. The mounting and acoustic condi¬ 
tions in the tube differ so much from those in 
the chamber that the final performance of the 
finished structure must also be carefully deter¬ 
mined. 

Measurements of the pressure-reflection co¬ 
efficient were made for more than 500 struc¬ 
tures of varying design. e The designs chosen 
were limited by the dimensions of the test 
apparatus at hand 32 ® to one single element with 
a base 8 in. square and to frequencies below 
1,500 c. Figure 74 gives the average and limits 



Figure 74. Pressure-reflection coefficient for 
P.F.-Fiberglas as installed in the form of 
wedges. (Average and limit values.) 


of the pressure-reflection coefficient of the struc¬ 
ture finally selected and taken from samples 
as delivered for installation. For this structure, 
the pressure-reflection coefficient stays below 
5 per cent from 100 c up. Measurements in the 
reverberation chamber show that this design 
has excellent absorption characteristics also at 
medium and high frequencies. 

From the large body of data on linear wedges 
of different dimensions 3211 generalized curves 
were constructed which permit the choice of a 
good structure if the cutoff frequency is given. 
This frequency is defined as the frequency at 
which the pressure-reflection coefficient rises 
to the value of 10 per cent. Three sets of con¬ 
tours are presented in Figures 75 through 77, 
relating to the linear dimensions, the flow re- 

e P ressur e-reflection coefficient, in per cent, equals 
100 VI —a where a is the energy-absorption coefficient. 
















































THE ANECHOIC CHAMBER 


169 


sistance per unit thickness (see Section 2.2.2), 
the volume density, and the cutoff frequency. 
These contours are useful, but it is necessary to 
use some caution in applying them. 



Figure 75. Design chart for P.F.-Fiberglas 
wedges. 











// 













/ 














r 

/ j 

V 













/ // 













4 

// 













A 

/ 











f i 

/ 

V 

// 

A 

'Ad 

AGE 










/ 

v 














A 













V 

























— 



/ 



































































2 3 4 5 6789 2 34567S9 

10 100 1000 
LOWER CUTOFF FREQUENCY, f c , IN CYCLES PER SECOND 


Figure 76. Design chart for P.F.-Fiberglas 
wedges. 


Figure 78 shows a sectional view of the large 
anechoic chamber at Harvard. The inside meas¬ 
urements of the reinforced concrete chamber 
are length, 50 ft 4 in.; width, 38 ft 4 in.; and 


height, 38 ft. The floor is 12 in. thick, and the 
portions of the walls located below ground have 
a thickness of 2 ft. Above ground, the walls are 
12 in. thick. 

Spool anchors were provided at intervals of 
4 ft to support the framework for the acoustic 
treatment. Anchors are also provided to hold 
the cables supporting the two-rail track ex¬ 
tending the length of the chamber, approxi¬ 
mately 12 ft above the floor (see Figure 72). 



1JD 10 100 

VOLUME DENSITY, LBS PER CU FT 


Figure 77. Design chart for P.F.-Fiberglas 
wedges. 


A layer of sheet cork was cemented to the 
walls and floor of the chamber with hot asphalt. 
This provides for moisture and thermal insula¬ 
tion. 

A set of carts can be rolled into the room to 
provide a floor. For many acoustic measure¬ 
ments the presence of the reflecting surfaces 
of the carts is undesirable. Rails are suspended 
from the ceiling so that test equipment can be 
supported without the presence of the carts. 

The treatment is installed by means of a 
wooden framework anchored to the walls of 
the chamber (see Figure 79). The air space 
behind the wedges is broken up by an “egg- 
crate” structure of Celotex Wall Board. The 
wooden frames of the wedges are nailed to the 
furring strips of the supporting framework in 
an interleaving manner. Supporting chains pre¬ 
vent the wedges from sagging. The edges of 



















































































































































170 


INTERPHONE COMPONENTS 


Figure 78. Sectional view of the large anechoic chamber at Harvard. 




Figure 79. Showing details of the mounting of the wedges. 































































































































































































THE ANECHOIC CHAMBER 


171 


adjacent wedges are at right angles to each 
other to reduce diffraction effects. Figure 80 
shows a portion of the wedges during instal¬ 
lation. 



Figure 80. Showing a portion of the wedges 
during installation. 


They consist of sheets of Transite f with the 
interior space packed with soundproofing ma¬ 
terial. As a result of this careful planning, the 
overall noise level in the chamber is very low, 
especially at night when heavy vehicular traffic 
on a nearby street has ceased. 



DISTANCE FROM SOURCE IN FEET 


The net working space after installation of 
treatment is 29x41x29 ft. 

As can be seen from Figure 78, only one 
opening is provided to connect the chamber 
with the outside. A set of three doors was con- 



Figure 81. Photograph of the inner doors with 
wedge treatment. 


structed to provide an effective acoustic lock. 
The innermost doors are lined with the same 
acoustic treatment as the rest of the chamber 
and are shown in Figure 81. The two outer sets 
of doors enclose between them a structure sep¬ 
arated from the rest of the building by small 
air gaps. The doors are fitted with heavy 
hinges, casters, and heavy compression clamps. 


Figure 82. Results of measurements of sound 
pressure vs distance in the large Harvard 
chamber (70 c). 



DISTANCE FROM SOURCE IN FEET 


Figure 83. Results of measurements of sound 
pressure vs distance in the large Harvard 
chamber (3,000 c). 


Complete air-conditioning and ventilating 
equipment is provided. The conditioned air 
enters the chamber through a removable plug 
in the acoustic treatment. Air is withdrawn 
from the chamber through a duct with intakes 
at the floor level behind the acoustic treatment. 
Provision is made to remove the intake and 
exhaust ducts leading from the chamber to the 
f A product of the Johns-Manville Company. 




































































































172 


INTERPHONE COMPONENTS 


air-conditioning unit and to seal all openings 
during measurements. 

Although the measurement of the absorption 
coefficient of the acoustic structure used in the 
chamber provides one means of assessing the 
performance of the structure, acoustic meas¬ 
urements in the finished chamber are also of 



DISTANCE FROM SOURCE IN FEET 


Figure 84. Results of measurements of sound 
pressure vs distance in the large Harvard 
chamber (10,000 c). 

interest. One of the simplest ways of judging 
how close the chamber approximates free-held 
conditions is to test the inverse square law. If 
a sound source is at hand for w r hich the sound 
pressure varies inversely with distance in free 
space, measurements of the sound pressure vs 



20 >00 >000 > 0.000 20,000 
FREQUENCY IN CYCLES PER SECONO 

Figure 85. Deviations for inverse square law 
for a commercial chamber. 

distance in the chamber under similar condi¬ 
tions will provide a useful index of perform¬ 
ance. A large number of measurements at dif¬ 
ferent frequencies for various orientations of 
source and microphone were taken at the Elec¬ 


tro-Acoustic Laboratory. Figures 82 through 
84 show a sample of the results of such meas¬ 
urements carried out at 70, 3,000, 10,000 c 
along one diagonal of the room. It can be seen 
that even for distances of 10 to 20 ft between 



Figure 86. Results of measurements of sound 
pressure vs distance in a chamber lined with 
wedges at the Psycho-Acoustic Laboratory. 


source and microphone, the deviations from 
inverse square law are small for all important 
frequencies. These data were taken with all 
carts removed from the room. For comparison, 
similar data are given on a somewhat smaller 
chamber described in reference 8 (see Figure 
85). 

































































































































































































THE ANECHOIC CHAMBER 


173 


Measurements of the same type in a much 
smaller chamber lined with wedges 15 in. long 
are given in Figure 86. A floor plan and interior 
view of this chamber, located at the Psycho- 
Acoustic Laboratory, are shown in Figure 87. 
The wedges are backed by 2 in. of rock wool, 




Figure 87. Floor plan and interior view of the 
chamber. 


which is, in turn, separated from the wall by 
an air space of 2 in. Figure 88 shows measure¬ 
ments of the sound pressure vs distance with 
the rock wool alone in place. Comparison with 
Figure 86 is instructive. 



Figure 88. Results of measurements of sound 
pressure vs distance before installation of the 
wedges. The acoustic treatment consisted of a 
layer of rock wool. 

























































































Chapter 11 

THE DESIGN AND DEVELOPMENT OF CERTAIN 
INTERPHONE COMPONENTS 


I T WAS perhaps inevitable that the testing of 
interphone performance and the standard¬ 
ization of test methods should have led to the 
development of new components, seeking to 
improve on the existing designs. At both the 
Electro-Acoustic and Psycho-Acoustic Labora¬ 
tories, considerable time has been spent in the 
modification and improvement of existing com¬ 
munication equipment. But in some cases the 
entire engineering job was assumed by the 
Harvard group, an activity which led to appli¬ 
cations for more than 65 different patents. The 
more important contributions made by the lab¬ 
oratories are briefly summarized in this chapter. 


n.i EARPHONE SOCKETS AND INSERT 
TIPS 

All headsets require some kind of a cushion 
or socket to couple the earphone to the ear. 
A satisfactory headphone socket must fit closely 
against the head all around the ear, must be 
made of sound-attenuating material, must en¬ 
close as small a volume as possible, and should 
be comfortable for the majority of the wearers. 
Sockets may fit into the ear (insert or semi¬ 
insert), over the ear (supra-aural), or around 
the ear (circumaural), but they must all meet 
these requirements. Some requirements are 
easier to satisfy with one type of socket than 
with another. Comfort is greatest with cir¬ 
cumaural sockets, the volume is smallest with 
insert tips, etc. 


1111 Circumaural Earphone Sockets 

Earphone cushions developed by the Harvard 
group were primarily of the circumaural and 
insert types. One of the earliest projects was 
concerned with the development of a circum¬ 
aural socket of the doughnut type (see Figure 
31 in Chapter 9) for helmets and headsets. 1 
The first design, shown diagrammatically in 


Figure 1, represents an attempt to use the 
doughnut construction in a very simple socket. 
The attachment of the doughnut to the socket 
allowed the unit to be used in headsets as well 
as in helmets. Increasing the weight of the rub¬ 
ber backing in order to realize greater protec¬ 
tion made the sockets too heavy for comfort. 
The excessive weight pulled a helmet away 
from the wearer’s head, and larger helmets were 
required to contain the socket. 

Accordingly, Design 1 was altered by re¬ 
ducing its weight and thickness. It was then 



Figure 1. Diagram of circumaural socket, Har¬ 
vard Design 1. Vertical and horizontal sections. 

found that this socket, while very comfortable 
for the wearer, left too large an air volume to 
be activated by the earphone. Further modifi¬ 
cations reduced this air volume, and provided 
a slight angle to fit the curvature of the head 
better. This socket, Harvard Design 5, is shown 
diagrammatically in Figure 2. The slight offset 
was satisfactory for use in helmets, but it was 
believed unsuitable for headset use because 
such a headset required the wearer to distin¬ 
guish between right and left sockets. Conse¬ 
quently, another socket, Harvard Design 6 (see 
Figure 3), which would fit on either ear was 
developed for headset use. 

These earphone sockets, when compared with 
most of the other sockets available at that time, 
afforded a substantial improvement in insula¬ 
tion without any sacrifice of comfort. The 
sockets were modified in minor details to suit 


174 








EARPHONE SOCKETS AND INSERT TIPS 


175 


specific applications, 9 but the general design 
was adopted and used in large quantities by 
both the Army and Navy. 

One of the inherent disadvantages of circum- 
aural sockets is the fact that they enclose a 
large volume of air, a factor counterbalancing 
their comfort and convenience. Harvard De¬ 
signs 5 and 6 suffer in this respect, especially 
as they do not realize the maximum reduction 
of the air volume possible with the circumaural 
type, and further developments were directed 




Figure 2. Diagram of circumaural socket, Har¬ 
vard Design 5. Vertical and horizontal sections. 


tained by using a relatively soft compound and 
by making that part of the socket which con¬ 
tacts the head thin and curved. A diagram of 
this socket, Harvard Design F-3R (Navy desig¬ 
nation CW-49506), is shown in Figure 5. 

The novel design of the headband also con¬ 
tributed to the thinness of the headset. The 
usual headband is attached to the back of the 
earphone and socket, but to save space the head- 
band for the “thin” headset supported the ear¬ 
phone and socket from the side. With this type 
of socket, the headband pressure necessary for 
good insulation is less critical than with the 
preceding types. 


111,2 Semi-Insert Tips 

The development of semi-insert tips presents 
no fewer problems than the design, construc¬ 
tion, and testing of the larger sockets. Neither 
does the semi-insert involve any principles which 
differ from those involved in the larger sockets. 
Instead of being seated around the ear, the 
semi-insert tips bear upon the surfaces around 
the orifice of the ear canal. The tip is main- 


toward reducing the volume by filling in the 
space behind the ear and by changing the plane 
of the face of the earphone to parallel the gen¬ 
eral forward surface of the pinna. The result of 
this work was Harvard Design 8-C, 6 shown dia- 
grammatically in Figure 4. A photograph of 
these sockets in a Canadian headset is shown in 
Figure 34 of Chapter 9. 

None of these earphone sockets is thin enough 
to fit under the Type M-l standard steel helmet, 
and at the request of the Marine Corps a thin 
circumaural socket was designed for use with 
a special thin earphone engineered at the Elec¬ 
tro-Acoustic Laboratory (see Section 11.2). The 
complete headset, adopted for Marine Corps 
use, is shown in Figure 16. 

This socket is interesting in that it is one of 
the first circumaural sockets used in this coun¬ 
try which gives moderately good protection 
from noise without the use of a doughnut. It is 
molded in one piece of relatively thin neoprene. 
The compliance necessary for comfort is ob- 



Figure 3. Diagram of circumaural socket, Har¬ 
vard Design 6. Vertical and horizontal sections. 

tained in position by the headband, as in the 
case of the larger sockets. The amount of pres¬ 
sure which can be tolerated is, however, con¬ 
siderably less than that used in the larger head¬ 
sets. The compliance necessary for fitting the 
semi-insert to the ear is obtained by shaping 















































176 


CERTAIN INTERPHONE COMPONENTS 


the end of the tip and by adjusting the thickness 
of the material. 

The amount of acoustic insulation afforded 
by a properly designed and fitted semi-insert 
tip is far in excess of that which can be expected 


Chapter 9), and at the same time to provide 
much greater comfort for the wearer. One of 
the successful models, Harvard Design C-6, is 
shown diagrammatically in Figure 6. The pres¬ 
sure required to produce a seal is very small, 



Figure 4. Diagram of circumaural socket, Harvard Design 8-C. Vertical and horizontal sections. 


of any of the sockets previously discussed. The 
attenuation of ambient sound is more nearly 
uniform for all frequencies, a condition which 
is ideally suited for the reception of desired 
airborne sounds (see Section 2.6) such as the 
speech of persons near by, while the exclusion 
of noise is highly favorable to the transduced 
signal. 

The earlier Harvard designs were intended 
to retain the excellent insulation of the Ground 
Forces semi-insert socket, M-300 (see Figure 32, 


and this design is considerably more comforta¬ 
ble than the M-300. 


Insert Tips 

The greatest insulation against ambient noise 
is provided by the “full-insert” tip. This type 
of socket is inserted in the ear canal, and sup¬ 
ports a miniature earphone without the aid of 
a headband. It also has the advantage that the 


























































EARPHONE SOCKETS AND INSERT TIPS 


177 


earphone operates into a minimum volume, 
thereby raising the effective sensitivity of the 
earphone. 


sible solution to this problem. Sanitary problems 
are similar to those encountered with earplugs 
(see Section 2.6). 



Figure 5. Diagram of circumaural socket, Har¬ 
vard Design F-3R. Vertical and horizontal sec¬ 
tions. 


A photograph of the Harvard Design M-9 
“Harvintip” is shown in Figure 7. 

Any device which is inserted into the ear 
canal creates a special problem in providing a 


111-4 Dual-Seal Socket 

Various attempts have been made to combine 
two of the generic socket types into a single 
unit. The Ground Forces headset H-16/U (see 
Figure 33, Chapter 9) illustrates the combina¬ 
tion of the circumaural and semi-insert designs 
into one socket. At the Electro-Acoustic Lab¬ 
oratory a cushion was developed which com¬ 
bined the circumaural and the supra-aural 
designs. A cross section of this dual-seal cush¬ 
ion is shown in Figure 8, and a photograph is 
given in Figure 9. The socket consists of two 
annular-shaped cushions. The outer cushion is 
designed to seal against the head around the 
ear, and the inner cushion approximates a seal 
against the pinna. By virtue of their thinness 
and flexibility the cushions may readily assume 
the shape of the head or ear. Tests indicate that 
the insulation of this socket is equivalent to 
that of the doughnut types, and the smaller 
volume of air which the headphone must acti¬ 
vate should increase the sensitivity of the 
headset. It should be noted, however, that a 



good fit to all ears. Several sizes must be pro¬ 
vided, and some care must be exercised in 
pushing the tip into the ear. Most users can be 
well fitted, but some observers find the Harvin¬ 
tip uncomfortable. Softer materials are a pos- 


highly refined design is required in order that 
both seals be effective on any large proportion 
of heads. Both of the above designs must be 
regarded as somewhat tentative in their present 
form. 









































































178 


CERTAIN INTERPHONE COMPONENTS 


112 HANDSETS AND EARPHONES FOR 
SPECIAL APPLICATIONS 

In spite of the tremendous advances of radio 
communication in World War II, the field tele¬ 
phone, consisting of two handsets, batteries, 
ringers, and connecting line, still played an 


By utilization of the driving unit of the 
ANB-H-1 earphone, the earphone is consider¬ 
ably reduced in size, and the shape of the hand¬ 
set was redesigned for most convenient accom¬ 
modation under the standard steel helmet. Fig¬ 
ure 11 shows a subject using an experimental 
model (Harvard Type MTS-1) of this design 
under the helmet. Note that the presence of the 
handset does not necessitate a sidewise move¬ 
ment of the helmet. Figure 12 shows photo¬ 
graphs of this handset taken from the front 
and side. An engineering drawing is shown in 
Figure 13. 

In addition to the obvious advantages of this 
design over the conventional types used in the 
standard field telephone (Type EE-8), the Type 
MTS-1 handset has rubber cushions placed over 
the earphone which make for a better seal at 
the ear. The push-to-talk switch was completely 



Figure 7. Harvintip insert tip, Harvard Design 
M-9. 


extensive and indispensable role. In funda¬ 
mental design, it was one of the best devices in 
the communication field. It had, however, one 
important defect. The handsets were of con¬ 
ventional design, similar to the standard tele¬ 
phone handset. As a result, they could not be 
used by the operator under the standard M-l 


NEOPRENE CUSHION 

METAL RING FELT FILLER—\ 



Figure 8 . Cross section of dual-seal cushion. 


steel helmet because of the bulky earphone. The 
helmet had to be cocked to the side, with a 
resultant decrease in protection (see Figure 
10 ). 

To overcome this difficulty, a handset of novel 
design was developed, based indirectly on a 
suggestion from captured German equipment. 


Figure 9. Photograph of dual-seal cushion. 

redesigned. The design was adopted in sub¬ 
stance by the U. S. Marine Corps. 7 

In order to provide privacy for the talker, 
which is necessary under certain conditions, a 
noise shield was added to the handset. A labora¬ 
tory model of a handset which uses an ANB-M- 
C1 mask microphone in a noise shield is shown 
in Figure 14, a photograph of the instrument in 
use position; Figure 15 shows photographs of 
the instrument itself. 7 " Difficulty in satisfac¬ 
torily fitting the wide range of face shapes and 
head sizes encountered led to the abandonment 
of the project. 

The early engineering work on the “thin” 























NOISE SHIELDS 


179 


earphone used in the CW-49507A headset which 
has been adopted by the U. S. Marine Corps 
(see Figure 16) was done at the Electro-Acous¬ 
tic Laboratory. 3 The magnetic structure of the 
ANB-H-1 earphone was redesigned to decrease 
the overall thickness at the expense of the 
diameter. The objective of accommodating the 
headset under the standard M-l steel helmet 
was thereby largely achieved. 

The enemy’s policy of instructing snipers to 
concentrate on communication personnel sug- 



Figure 10. Conventional field telephone. Hand¬ 
set cannot be used effectively under standard 
steel helmet. 


gested that some advantage might be gained 
by devising an earphone small enough to be 
concealed in the ear itself. Such an ultra¬ 
miniature earphone was under development by 
the Permoflux Corporation at the end of World 
War II. Approximately a centimeter long and a 
centimeter in diameter, the earphone itself is 
even smaller than the insert tip designed for it. 
When the wearer is viewed face-on, the ear¬ 
phone is not visible in his ear. A photograph of 
the experimental unit is shown in Figure 17. 


113 NOISE SHIELDS 

The most ambitious attempt to improve the 
discrimination between speech and noise at the 
microphone end was in the construction of 
noise shields which used actual mechanical 
shielding to exclude noise from the microphone. 
Several laboratories have tried their hands at 
finding a satisfactory design. At the Psycho- 
Acoustic Laboratory, 25 different models were 
tried at various times. The acoustical problems 



Figure 11. Handset designed for use under 
standard steel helmet. 


of noise exclusion and satisfactory frequency 
response were fairly well solved in the earlier 
models, but further effort was expended in an 
attempt to increase the comfort and to simplify 
the construction of molds for production pur¬ 
poses. 

One of the earlier models, Harvard Design 
D-ll shown in Figure 18, indicates the main 
features which were incorporated. The noise 
shield holds an oxygen-mask microphone and 
provides a leak for the expired breath. It is 








180 


CERTAIN INTERPHONE COMPONENTS 



Figure 12. Photograph of model of improved 
handset. 


held against the face by straps around the head. 
The cavity size was larger than necessary, and 
the face fit rather unsatisfactory. Both were 
rectified in the more conical shape of Harvard 
Design D-17, shown in Figure 25 of Chapter 9. 
Military personnel who used these models com¬ 
plained about having a device of this kind in 
contact with their faces, and the designers ex¬ 
perienced difficulties in getting molds made 
properly. Although considerable interest was 
aroused in the D-17 design, it was never used 
for other than training purposes, and efforts 
to produce a noise shield were finally abandoned 
when the noise-canceling microphone appeared 
to meet the more pressing needs of the Air 
Forces. 

Canadian interest in the noise shield resulted 
in the development of a model for use with a 
hand-held microphone. It was adopted for use 
with a dynamic microphone (see Figure 19). 
In the design of this shield an attempt was made 
to get a good seal against the face and yet retain 
a very simple conical construction. A diagram 



ITEM 

NAME 

MATERIAL 

1 

HANDLE 

ALUMINUM 

CASTING 

2 

REAR COVER 


3 

RECEIVER CAP 

" 

4 

TRANSMITTER CAP 

" 

5 

SWITCH BUTTON 

PHENOLIC 

MOLDING 

6 

RECEIVER CUSHION 

NEOPRENE 

GN 

7 

TRANSMITTER CUSHION 


8 

SWITCH MOUNTING 

PHENOLIC 

9 

SWITCH GASKET (CEMENTED 

TO SWITCH MOUNTING) 

BUNA"S" 

10 

SWITCH BUTTON STOP 

PHENOLIC 

11 

REAR COVER GASKET 
(CEMENT TO HANDLE) 

BUNA "S" 

12 

TRANSMITTER GASKET(CEMENT 
TO FACE OF TRANS. CAVITY) 

NEOPRENE 

14 

RECEIVER GASKET(CEMENTED 

TO FACE OF RECEIVER CAVITY) 


15 

SPRING a CONTACT ASS'BLY 


17 

CONTACT DISC ASSEMBLY 

- 

19 

CORD ASSEMBLY 

- 

20 

TRANSMITTER ASSEMBLY 

- 

21 

RECEIVER ASSEMBLY 

- 


Figure 13. Drawing of improved handset. 








































































NOISE SHIELDS 


181 



Figure 14. Photograph of handset used with 
noise shield. 




Figure 16. Photograph of headset assembly 
Navy Type CW-49507A (U. S. Marine Corps). 



Figure 17. Photograph of ultra-miniature ear¬ 
phone with insert tip. 


Figure 15. Photograph of model of handset with 
noise shield. 



















182 


CERTAIN INTERPHONE COMPONENTS 


of the noise shield, Harvard Design D-25, is 
shown in Figure 20. The real-voice response of 
the Canadian unit is shown in Figure 21. 

An air vent is, of course, a necessary feature 
of a noise shield and may reduce the mask’s 
noise-excluding properties unless special care is 
taken in designing the opening. The Harvard 
designs successfully employed several rather 
long vents of small diameter. Such vents pre- 


more resistive in nature. The inclusion of the 
nose under the mask, as in the Bell Telephone 
Laboratories design, has the advantage of in¬ 
cluding the nasal speech sounds. If the noise 
levels in aircraft continue to rise, it may even¬ 
tually become necessary to adopt an improved 
shield as the only feasible method of obtaining 
the speech-to-noise ratio necessary for success¬ 
ful communication. 



sent little resistance to the direct flow of air 
but a high impedance to the passage of sound. 

The optimum performance of a noise shield 
has probably not been obtained in any of the 
designs discussed above. The present limitation 
is almost certainly the severe acoustical reac¬ 
tion on the voice of the small cavity of the 
shield. Further improvements in the acoustical 
properties of noise shields will probably result 
from making the acoustic impedance lower and 


11 ‘ OXYGEN MASKS 

Some of the fundamental properties and 
characteristics of performance of oxygen masks 
have been discussed in Sections 9.2.4 and 10.2.1. 
In this section, certain tests made on an arti¬ 
ficial voice are discussed which may be helpful 
in design problems involving oxygen masks. At 
the outset, it must be emphasized that the 
fundamentals of the performance of enclosures 
coupled to the mouth are, as yet, imperfectly 










































OXYGEN MASKS 


183 



Figure 19. Canadian hand-held dynamic micro¬ 
phone with Harvard Design D-25 noise shield. 

understood. Many of the results must be inter¬ 
preted with this fact in mind. 


BOTTOM 






Figure 20. Drawing of noise shield, Harvard 
Design D-25. 


Early experiments with small enclosures 
coupled to an acoustic source of high internal 
impedance 2 have demonstrated two things. 


First, the response of a mask microphone per se 
must have a rising frequency-response charac¬ 
teristic at low and medium frequencies in order 
that a satisfactory response characteristic of the 
mask-microphone combination be realized. Sec¬ 
ond, the distribution of sound pressure in the 
mask becomes nonuniform at medium and high 
frequencies and proper microphone location 
within the mask is important. 

In addition, many experiments have been 
carried out in order to determine the effects of 
the oxygen supply hose, mask size and seal 
to the face, the noise exclusion properties of 
oxygen masks, and similar factors. Some of the 
relevant data are presented in report form. 5 



FREQUENCY IN CYCLES PER SECOND, 

Figure 21. Real-voice response of Canadian 
ZA/CAN 5155 hand-held microphone with Har¬ 
vard Design D-25 noise shield. 


There remains a pressing need for considera¬ 
tion of acoustical factors at an early stage in 
the development of any future masks. 

While the artificial voice is a basic tool in the 
hands of the designer, its chief merit lies in its 
simplicity and the speed with which comparative 
results can be obtained. Real-voice response 
measurements are an important supplement 
and should also be utilized. But certain acous¬ 
tical properties of oxygen masks and noise 
shields, as yet improperly understood, cause 
speech produced within them to be seriously 
distorted. In many cases, significant differences 
in articulation scores exist between masks 
which real-voice response characteristics fail 
to predict. Formal articulation tests remain, 
therefore, the only method which will give a 
high correlation with actual performance. 









































































184 


CERTAIN INTERPHONE COMPONENTS 


115 GAS MASKS 

At the request of the U. S. Army Chemical 
Warfare Service, an investigation of the acous¬ 
tical properties of gas masks was undertaken 


types soon became apparent. 4 Masks of later 
design were equipped with a diaphragm (speech 
transmitter) which was frequently combined 
with the exhaust valve (angle tube). Figure 22 
shows an early mask without speech transmit- 



Figure 22. Gas mask without diaphragm. 


Figure 23. Gas mask with diaphragm for im¬ 
proved speech transmission. 


by the Harvard group. These properties are 
important in two related ways. First, it is nec¬ 
essary for combat personnel exposed to gas to 
communicate with each other directly; second, 
successful speech transmission over field tele- 


ter. A mask which utilizes an angle tube (com¬ 
bination diaphragm and exhaust valve) and 
which was proposed for use early in 1944 is 
shown in Figure 23. 

Articulation tests 4 have shown that masks 



Figure 24. Photograph of laboratory model of 
gas mask with improved speech transmitter. 


V ' flOOF ASSEMBLY : 



.DIAPHRAGM support 


Figure 25. Photograph of speech transmitter 
parts (Harvard Design). 


; DIAPHRAGM).. 




phones or over battle telephones on shipboard 
is necessary. 

In the masks intended for use at the begin¬ 
ning of World War II, the exhaust valve was 
the chief path of acoustic transmission to the 
outside. The acoustic ineffectiveness of these 


equipped with diaphragms are acoustically su¬ 
perior to masks relying on the exhaust valve 
alone for speech transmission. However, the 
articulation scores for all masks tested were 
substantially lower than the scores obtained 
for talkers without masks. This was found to 







GAS MASKS 


185 


be true for articulation tests conducted in a 
low ambient noise as well as for tests conducted 
outdoors. 



'DDDCC /" 

CCD 

^* 1 ^. 

n i p-| tjrj 111 nj 11 11 111j i |V| > 1 n i 


Figure 26. Photograph of speech transmitter 
assembled (Harvard Design). 

In articulation tests where the person wear¬ 
ing a gas mask tried to use the standard field 


to the exhaust valve or diaphragm. Since this 
necessitated the removal of the earphone from 
the ear, it became clear that a complete rede¬ 
sign of mask (rather than of the telephone) 
was in order. This was equally evident after a 
series of experiments attempted to improve 
speech transmission into open air by minor 
modifications of the diaphragm mask. 

Physical tests were performed using real and 
artificial voices, 43 but it soon became evident 
that these tests did not tell the whole story, 
since interaction between the speech mechanism 
and the mask was an important, if not control¬ 
ling, factor. The phenomena appear even more 
complicated than those present in noise shields 
and oxygen masks, since gas masks involve 
sound transmission through the mask to the 
outside. 

During 1945, an attempt was made to design 
a new speech diaphragm which, in conjunction 
with the mask, would provide more effective 




Figure 27. Photograph of boom mounting of lip 
microphone in use position (front view). 

telephone, the observation was made that speech 
transmission was intolerably poor unless the 
microphone of the handset was held very closely 


Figure 28. Photograph of boom mounting of lip 
microphone in use position (side view). 

speech transmission into open air and over 
standard field telephones. 

Figure 24 shows a laboratory model of the 
















186 


CERTAIN INTERPHONE COMPONENTS 


design which was finally evolved. It shows the 
speech transmitter (E17R8) mounted on a face- 
piece with lateral canister. 8 Figure 25 shows 
the parts of the speech transmitter. The active 
part of the valve is a rubber strip of V-shaped 
cross section, with semicircular diaphragms 
extending from it. The valve rests against the 
valve seat, whereas the diaphragm is con¬ 
strained by the metal diaphragm support. The 



Figure 29. Photograph of boom mounting of lip 
microphone in standby position. 


structure is protected from the outside by a 
metal roof. The assembly is held together by a 
crimped edge (see Figure 26). Air pressure 
from the inside of the mask opens the valve. 
The perforated metal diaphragm support fur¬ 
nishes effective protection against gun blast, 
whereas the roof provides protection against 
diaphragm puncture and foreign matter. 

Articulation tests have shown that a mask 
equipped with the new E17R8 speech trans¬ 
mitter is somewhat superior to older diaphragm 
masks. Performance over field telephones should 
be markedly better, since it is no longer neces¬ 
sary to remove the handset from the ear in 


order to place the microphone against the open¬ 
ing in the mask. But despite the fact that a 
significant improvement has been realized, the 
basic problems have not been solved. 

An attempt to attack the problem of inter¬ 
action between mask and voice in a thorough 
way has been made. 8a The effects of small, ideal¬ 
ized enclosures terminated in openings of vari¬ 
ous sizes has been studied. An artificial voice 
was used, similar to the one discussed in Chap¬ 
ter 10 (shown in Figure 28 of that chapter). 
It was transformed effectively into a constant 
current source by packing the tube simulating 
the mouth with wires. The sound pressure de¬ 
veloped in the cavity was measured by means 
of a probe microphone and was taken to be 
proportional to the acoustic impedance of the 
enclosure. By comparing the sound pressures 
inside and outside the cavities, an index of the 
transmission of sound was obtained. The effect 
of such enclosures on the live voice was also 
studied, and many interesting results have 
been obtained. sb Much research of a funda¬ 
mental nature, however, remains to be done 
before the mechanism of interaction between 
voice and mask is understood. 


116 BOOM MOUNTING FOR 

MICROPHONES 

The advantages and favorable performance 
data of the “lip” microphones of the noise¬ 
canceling type have already been pointed out 
(see Section 9.2.2). A photograph of the Navy 
noise-canceling microphone M-5/UR with har¬ 
ness is shown in Figure 16 in Chapter 9. As a 
result of field tests, certain objections to this 
type of suspension were raised, a summary of 
which follows. 

1. The harness, in direct contact with the 
lips, is uncomfortable and might cause skin 
irritation, especially in the tropics. 

2. The harness sometimes causes pressure of 
the microphone against the septum of the nose, 
a further cause of discomfort. 

3. No provision is made for a convenient 
standby position. 

4. Large accelerations in pull-outs tend to 
displace the microphone down over the mouth. 







BOOM MOUNTING FOR MICROPHONES 


187 


To correct this situation, the U. S. Navy- 
Bureau of Aeronautics encouraged the develop¬ 
ment of an alternative mounting. A solution 
was found in the form of a boom mounting. 
The microphone is supported in a boom made 
of stiff wire which is supported on a pivot on 
the earphone cushion. Photographs of this 
mounting are shown in Figures 27 and 28. The 


boom in standby position is shown in Figure 
29. Various orientations of the microphone 
button with respect to the boom were tried and 
the performance was measured in terms of ar¬ 
ticulation score and noise pickup (see Section 
10.1.2). 7b These mountings were available for 
use by the U. S. Navy at the end of World 
War II. 



Chapter 12 

SPECIAL VOICE COMMUNICATION SYSTEMS 


121 SOUND-POWERED TELEPHONE 
SYSTEMS 


A SOUND-POWERED telephone system is an 
interphone system (see Figure 3, Chapter 
9) without an amplifier or any other source of 



Figure 1. Photograph of Type M sound-powered 
telephone instruments (U. S. Navy). 


electric energy. The energy necessary to oper¬ 
ate the system is supplied solely by the talker’s 
voice. The separate lines necessary in a con- 



Figure 2. Photograph of sound-powered handset 
(U. S. Navy). 

ventional interphone for headsets and micro¬ 
phones are combined, and the network connect¬ 
ing the stations is a simple two-wire line. 

Sound-powered telephones are used most 
extensively aboard ships. A great number of 


circuits are provided, and the number of sta¬ 
tions per circuit is generally large, sometimes 
exceeding twenty-five. Sound-powered handsets 
have also been used, to some extent, as field 
telephones by the Ground Forces. 

On shipboard the bulk of the interior com¬ 
munications during General Quarters is carried 
by sound-powered telephones. The operators 
manning the stations consist of enlisted per¬ 
sonnel equipped with headsets and microphones 
mounted on chest plates. Figure 1 shows an 
instrument of this type (Type M) using ear¬ 
phone cushions with a supra-aural seal. These 
sound-powered telephones were in use through¬ 



Figure 3. Photograph of Type 0 sound-powered 

telephone instruments (U. S. Navy). 

out World War II. Certain key officer personnel 
below deck, for example, make use of sound- 
powered instruments combined in a handset 
(see Figure 2). 

At the close of the war, headsets of Type M 
were being replaced by units of decreased size 
and weight (see Figure 3) utilizing removable 
semi-insert tips (Type 0). This change was in¬ 
tended to overcome some of the criticisms 
leveled against the older headsets, such as their 
excessive weight and bulk, the necessity of 
using a special large helmet at exposed stations, 
and the discomfort which resulted from pro¬ 
longed wear. While these objectives were 
achieved in some measure, the performance of 
Type 0 headsets fell below that of the Type M 
in certain other important respects. Extensive 


188 











SOUND-POWERED TELEPHONE SYSTEMS 


189 


articulation tests in noise showed that the 
Type 0 units were comparable to Type M units 
only if used with the tips, and then only if 
great care was taken to obtain as good a seal 



Figure 4. Orthotelephonic response of a typical 
sound-powered telephone system (Type M). 


on the ear as possible. The performance of the 
Type O units, used without the tips, was shown 
to be definitely inferior. 4 - 5 - 9 Reports from the 
Fleet indicate that there is little likelihood that 


designs used or proposed for use during World 
War II, quantities of more general information 
have been gathered. The remainder of this sec¬ 
tion is devoted to a discussion of some of the 
general properties of sound-powered telephone 
systems and their performance in noise. For 
simplicity, Type M units will be assumed 
throughout the discussion. 

The efficiency of sound-powered earphones 
and microphones in transducing sound into 
electric energy and in again converting the 
electric energy back into sound must be made 
high to compensate, if possible, for the lack of 
external amplification. This has been achieved 
to a considerable degree in practical designs 
but only at the expense of the bandwidth of the 
system. Figure 4 shows the overall orthotele¬ 
phonic gain of a typical sound-powered tele¬ 
phone system. It can be seen that this system 
transmits efficiently only a relatively narrow 
band of speech, about one octave wide, centered 
about a frequency of 1,500 c. The maximum 
orthotelephonic gain, near 1,500 c, is about 20 
db, i.e., the level of received speech is 20 db 
higher than it would be over an air path of 1 m. 



100 ’OOO 10,000 

FREQUENCY IN CYCLES PER SECOND 


Figure 5. Real-voice response of a typical 
sound-powered microphone (Type M). 


“ If 
*S 
- 8 

§* 
!2 z 





























































































/"N 














:7 ^ 















\ 














V 














\ 















YU i 



1000 

FREQUENCY IN CYCLES PER SECOND 


5 6 7 8 9 

10.000 


Figure 6. Real-ear response of a typical sound- 
powered earphone (Type M). 


the tips will be properly used or used at all by 
combat personnel. 

Further performance data (response and in¬ 
sulation) of Type 0 headsets are given else¬ 
where. 7 - 12 It is understood that the U. S. Navy 
Bureau of Ships is now procuring instruments 
of novel design which may overcome some of 
the present difficulties. 

In the course of testing the many particular 


The rapid decrease in the response above 
1,500 c is due to both earphone and microphone 
(see Figures 5 and 6). The decrease in the re¬ 
sponse at the lower frequencies is due mainly 
to the microphone. 

It is evident at once from Figure 4 that the 
effective pass band of such a system, i.e., the 
band of frequencies where the speech-to-noise 
ratio is contributing significantly to the articu- 









































































































190 


SPECIAL VOICE COMMUNICATION SYSTEMS 


lation index (see Chapter 9), is a function of 
the noise level, since there is a quite pronounced 
upper limit to the speech level a talker can 
maintain comfortably. In noise, a band of fre- 



Figure 7. Gain function obtained with a syn¬ 
thetic sound-powered system in noise. For com¬ 
parison, gain functions for two band-pass systems 
with sharp cutoffs are shown also. 

quencies even less than an octave wide may 
be effective, whereas in quiet surroundings the 
effective band will exceed two octaves. If artic¬ 
ulation tests in varying levels of noise are 


widths. Such tests have been performed using 
a synthetic sound-powered system closely ap¬ 
proximating the real one. A noise spectrum was 
used resembling the spectrum in a submarine 



Figure 8. Showing the performance of a typical 
sound-powered telephone system in ambient noise. 


engine room. Then, with the talker located in a 
quiet place and speaking in a loud voice, the 
articulation functions shown in Figure 7 were 
obtained. Although, for these tests, the gain 



Figure 9. Showing the influence of the spacing 
between the microphone and the talker’s lips for 
a typical sound-powered telephone system. 


performed on such a system, it is possible to 
“bracket” the resulting function by correspond¬ 
ing gain functions (see Chapter 5) of two sys¬ 
tems with sharp cutoffs and suitable band- 



Figure 10. Showing the influence of an “open” 
microphone on the performance of a typical 
sound-powered telephone system. 


of the synthetic system was varied by means 
of an amplifier, essentially the same func¬ 
tions would have been obtained if the noise 
levels had been changed in an inverse propor- 

























































SOUND-POWERED TELEPHONE SYSTEMS 


191 


tion while an amplifier of fixed gain (zero) was 
being used. The functions for the two “bracket¬ 
ing” systems with sharp cutoffs are also shown. 
From these functions, it can be concluded 



Figure 11 . Showing the improvement in articu¬ 
lation score afforded by the use of a noise shield 
over the sound-powered microphone. 


that the performance of a typical sound- 
powered telephone system is adequate in rela¬ 
tive quiet only. Under some noise conditions 
encountered on shipboard the performance 



Figure 12. Noise-canceling carbon microphone 
Type M-6/UR with adapter kit for use on a 
sound-powered telephone line. 


breaks down. This has been confirmed by analy¬ 
ses of recordings taken from actual sound- 
powered circuits while they were in normal 
use. 3 A somewhat more precise demonstration 
of the effect of noise in limiting actual per¬ 


formance is afforded by laboratory articulation 
tests. Figure 8 shows the articulation scores 
obtained when both listener and talker are 
immersed in “engine-room” noise. At the level 
actually found, for example, in a submarine 
engine room, the low articulation score indi- 



Figure 13. Showing the improvement in articu¬ 
lation scores obtained by using a noise-canceling 
microphone in a sound-powered telephone system. 


cates that the system will fail seriously because 
of the many necessary repeats. 

With the talker alternately in quiet and in 
the same noise as the listener, articulation 
scores obtained on a sound-powered system in- 



2 3456789 2 3 456789 

100 1000 10,000 
FREQUENCY IN CYCLES PER SECOND 


Figure 14. Acoustic insulation provided by 
sound-powered earphone (Type M). 

dicate that the noise picked up by the micro¬ 
phone contributes significantly to the masked 
threshold in the pass band. This is true even 
for close-talking conditions. As the number of 
headsets is increased, the relative contribution 
























































192 


SPECIAL VOICE COMMUNICATION SYSTEMS 


by the microphone to the total masking de¬ 
creases, and, for a very large number of head¬ 
sets across the line, the noise leaking past the 
earphone cushion is the controlling factor. 

The sound-powered telephone system per¬ 
forms satisfactorily under conditions of rela¬ 
tive quiet. Furthermore, such a system is the 
simplest one possible. Its reliability is high if 
it is properly designed. It is less vulnerable 
than a system with central batteries, although 
the present-day practice of terminating many 
lines in the switching facilities of the Internal 
Communications Room aboard ship makes it 
more vulnerable to a “lucky” hit than would 
appear at first glance. 

On the other hand, there is ample evidence 
that the sound-powered system fails under the 
stress of ambient noise. This is due not only 
to the properties of the system per se, as shown 
above, but can also be attributed in no small 
measure to the present doctrine of using tele¬ 
phone “talkers.” More often than not, the 
telephone line terminates in a “talker” rather 
than the man who is charged with executing 
the transmitted order. The talker’s function is 
solely that of a relay, transmitting and receiv¬ 
ing messages in open air to and from a second 
person who receives or gives the order. This 
procedure may be necessary in many instances; 
it is as often unnecessary and ineffective. The 
talker is often the weak link in the system, for 
in noise he has difficulty in hearing the spoken 
order while he is wearing the sound-powered 
headset. Correspondingly, it is difficult for the 
talker to shout the message through the open 
air. Detailed studies have shown that on some 
circuits, mainly those concerned with fire con¬ 
trol, the number of open-air links is far too 
large for satisfactory performance. 

Adequate training and correct attitudes of 
personnel are important. Good enunciation, 
high speech levels, and use of appropriate 
speech material are helpful. The microphone 
should be held as closely as practicable to the 
talker’s lips. Figure 9, for instance, shows the 
drop in articulation scores as the distance be¬ 
tween microphone and the talker’s lips is in¬ 
creased. The microphone press-to-talk switch 
should be operated only if a message is to be 
transmitted. One “open” microphone on the 


line has the effect shown in Figure 10. All head¬ 
sets connected to the line should be worn with 
as good a seal as possible at all times. 

It seems probable that a proper study of the 
systems and circuits and coordination of this 
study with improved doctrine would outline 
improvements which would raise the operating 
efficiency of intraship circuits. Then, as a 
further step, the possibilities of improving the 
sound-powered system proper should be inves¬ 
tigated. 

The signal-to-noise ratio provided by pres¬ 
ent-day sound-powered microphones can be 
improved by the use of a noise shield. A labora¬ 
tory model of such a shield was produced by 
modification of the Harvard D-17 noise shield 
to fit the sound-powered microphone (see Sec¬ 
tion 11.3). Figure 11 shows the improvement 
in articulation scores resulting from its use. 

As a further step, use might be made of 
carbon microphones of the noise-canceling type. 
An adapter kit 11 (see Figure 12) was devised 
utilizing a Type M-6/UR lip microphone 
mounted on a boom suspended from the ear¬ 
phone. The kit includes the necessary dry cells, 
transformer, and switch. The improvement in 
articulation score resulting from the use of the 
adapter is quite striking (see Figure 13). 

Tests show that the Type M headset excludes 
noise quite well. To achieve this, however, the 
headband exerts a large pressure on the ears, 
and the earphones are of considerable weight 
(see Figure 14). The sensitivity compares 
favorably with other types of headset. The fre¬ 
quency responses of sound-powered micro¬ 
phones and earphones (see Figures 5 and 6) 
show that, at present, the bandwidth of the 
system is largely limited by the microphone. 
It seems unlikely that the efficiency can be 
further increased without narrowing the pass 
band beyond tolerable limits. Manufacturing 
tolerances, available magnetic materials, im¬ 
pedance mismatch between microphone and 
earphone, and rigid specifications for resistance 
against corrosion and blast set severe require¬ 
ments for practical designs. 

The question arises, therefore, whether 
present-day response curves constitute the 
optimum consistent with current design re¬ 
quirements. Suppose a system were built with 





PORTABLE RADIO EQUIPMENT 


193 


a pass band one octave wide but centered 
around 1,000 c instead of 1,500 c, the orthotele- 
phonic gain being the same in both cases. How 
would the performance of these two systems 
compare? Such questions can be answered in 
general by making use of the generalized equal- 
articulation contours (see Section 7.2.4) ob¬ 
tained from a thorough study of the articulation 
efficiency provided by bands of speech in noise. 
From the more detailed discussion presented 
in an earlier report, 6 it can be concluded that 
no net gain in performance will result from the 
shift in frequency mentioned above. A sig¬ 
nificant improvement will be obtained only if 
the orthotelephonic gain or the bandwidth or 
both are increased. 

With these severe limitations imposed on a 
sound-powered system, the question arises 
whether it may not be more profitable to aban¬ 
don the sound-powered principle for the battle 
telephone, or at least relegate it to its original 
standby status, and install an interphone sys¬ 
tem using efficient wide-band instruments and 
electronic amplifiers. The idea of using a 
central amplifier for each circuit, similar to the 
interphone system used in aircraft, seems 
attractive. On the other hand, individual sta¬ 
tion-amplifiers could be built. The problem is 
a difficult one to solve in practice, especially 
when questions of doctrine, procurement, and 
investment in existing facilities must be bal¬ 
anced against the desired performance. It is 
hoped that a thorough survey will be under¬ 
taken to re-evaluate the present system of 
internal communications over sound-powered 
telephones, and that this, supplemented by suit¬ 
able research, will result in recommendations 
for future designs. 


122 PUBLIC-ADDRESS SYSTEMS 

Sound systems of the public-address type 
have been used for a variety of purposes in 
World War II. On shipboard, battle-announcing 
systems are in use to transmit general an¬ 
nouncements and orders, calls to battle stations, 
and certain special code signals. On carriers, 
instructions and orders are transmitted to the 
hangar and flight decks. 


On the ground, sound systems are used to 
direct certain battle and landing operations 
and transmit propaganda to nearby enemy in¬ 
stallations. Some of the systems used by the 
U. S. Marine Corps have been tested. 10 The 
more important performance parameters are 
the maximum undistorted acoustic output, the 
frequency-response characteristic, directivity, 
and nonlinear distortion. 


12.3 PORTABLE RADIO EQUIPMENT 

The overall response characteristic of a 
communication system is obviously the cumula- 



Figure 15. Portable radio transmitter-receiver. 


tive result of the response characteristics of 
its components. If several of the components 
pass only relatively narrow bands of frequen¬ 
cies, the total effect is sometimes striking. The 
performance is sometimes determined by the 
single component with the narrowest pass band. 
A case in point is the sound-powered telephone 
system. Another instance was provided by 
portable radio equipment. This was brought to 
the attention of the Harvard group in 1943. 1 
The system in question, used extensively in 
World War II, is shown in Figure 15. Upon 
investigation, it turned out that the earphone 
and microphone were the limiting elements 
in determining the quite unsatisfactory overall 
frequency response. There is evidence that, 
as is too often the case, the designers of the 
various equipments went their own way with¬ 
out much regard for coordination and overall 
performance. By substituting more suitable 
earphones and microphones, the overall fre¬ 
quency response was improved, as shown in 
Figure 16. 





194 


SPECIAL VOICE COMMUNICATION SYSTEMS 


12 4 COMMUNICATION SYSTEM FOR 
SHALLOW-WATER DIVING 

The recent development of improved and 
simplified equipment for shallow-water diving 



too 


5 6 7 0 9 

J000 


3 4 5 6 7 8 9 

10,000 


FREQUENCY IN CYCLES PER SECOND 


Figure 16. Improvement in overall frequency 
response of the radio transmitter-receiver by the 
use of a more suitable earphone and microphone. 


two divers, operating in shallow water, to 
communicate with each other and with the 
tender. A set of earphones and a microphone 
are provided for each of the three operators, 
and the three are linked together by a portable, 
battery-operated amplifier and suitable cables. 

Photographs of the diving suit before in¬ 
stallment of the headset and microphone are 
shown in Figure 17. It consists of a watertight 
diving dress which covers all but the diver’s 
face. A mask is strapped over the diver’s face 
with a triangular plastic window in front. An 
intake control valve is provided for air which 
is supplied from a tank on the tender. The 
suit was modified to accommodate in the proper 
position two earphones sheathed in rubber. A 
special socket was devised to house an earphone 
unit similar to the one used in the Marine 
headset CW-49507. The leads are of a special 
type and are provided with a special air duct 
to achieve pressure equalization between the 



Figure 17. Photograph of a diving suit designed for operation in shallow water. No communication 
equipment is as yet installed. 


led the U. S. Navy Bureau of Ships to request ears and the mask cavity. All junction points 
assistance in the design of a communication are carefully made watertight, 
system suitable for use in improved diving The microphone used in the mask is of the 
dress. The system which was developed 8 enables noise-canceling type (CW-51066). It is mounted 












































COMMUNICATION SYSTEM FOR SHALLOW-WATER DIVING 


195 


on an adjustable bracket. Although the micro¬ 
phone was originally designed for operation 
in open air, its use in an enclosure does not 



Figure 18 . Sketch of diving suit and mask 
fitted with earphones and microphone. 



Figure 19. Photograph of headset and micro¬ 
phone used by tender. 


result in an increase in the low-frequency re¬ 
sponse, as would be the case for ordinary 


microphones. This is due to the fact that, at low 
frequencies, the pressure gradient in the mask 
is comparable to that in open air, so that the 
response of the lip microphone in the mask 
is not greatly increased over what it would be 
in open air. a It was found necessary to reduce 
the noise generated by the air flow to a tolerable 
magnitude by installing a suitable silencer. A 
sketch of the diving suit and mask fitted with 
the communication instruments is shown in 
Figure 18. The tender uses ANB-H-1A ear¬ 
phones and a noise-canceling microphone with 
boom suspension (see Figure 19). 

A photograph of the amplifier is shown in 
Figure 20. It is battery-operated and contains 
carbon amplifiers and the necessary switches 
and cable connections. Its weight is approxi- 



Figure 20. Carbon amplifier for communication 
with two divers operating in shallow water. 


mately 25 lb. The overall frequency response 
of the system is essentially uniform to about 
3,000 c. The change in response resulting from 
the increased ambient pressure at a depth of 
60 ft has been measured and found to be not 
significant. 

Figure 21 shows an early laboratory model 
of the modified diving dress with a junction box 
atop the diver’s head. This design was later 
abandoned in favor of the arrangement 
sketched in Figure 18. 


a Noise-canceling lip microphones have also been used 
successfully in oxygen masks. 





















196 


SPECIAL VOICE COMMUNICATION SYSTEMS 



Figure 21. Showing experimental installation of 
earphones in the diver’s suit. (The junction box 
was later abandoned. See Figure 18.) 


A laboratory model of the equipment has 
been repeatedly tested in a pressure tank and 
has met with general approval by the Navy. 


123 VOICE TUBES IN SUBMARINES 

In spite of all the advances made in the 
communication art, the voice tube has remained 
the unchanging standby system for use when 
all others fail. In some instances, such as be¬ 
tween the bridge and conning tower of sub¬ 
marines, communication proved to be very 
ineffective. After extensive tests had been con¬ 
ducted, it was shown that proper design of the 
tubes and junctions, the presence and design 
of valves in the voice-tube circuit, and appropri¬ 
ate choice of horns at the talking and listening 
ends are all of critical importance. The sub¬ 
marine differs from other ships only in the 
severity of the space requirements, leading to 
improper location of lines, and number of 
valves required on lines passing through the 
hull. Full results and recommendations are 
presented elsewhere in report form. 2 






Chapter 13 

NOISE REDUCTION IN RADIO RECEIVERS 


T he radio link, like the interphone, is often 
the victim of interfering noise, and satis¬ 
factory speech communication depends in large 
measure upon the establishment of a favorable 
signal-to-noise ratio. Atmospheric static is usu¬ 
ally the greatest problem to be overcome, 
although ambient noise, precipitation static, 
interference from radar modulators and elec¬ 
tronic altimeters, ignition noise, interference 
from electric motors, interchannel interference 
or deliberate jamming, and set noise from the 
receiver itself are all too often encountered. 


131 STATIC 

A large part of atmospheric static, like most 
of the specific interferences mentioned, belongs 
to a class of interference which may be referred 
to as impulse noise. The intense static from 
local thunderstorms, and probably most other 
types of atmospheric static, arise from very 
brief electric or electromagnetic disturbances. 
These disturbances frequently occur in bursts, 
trains, or crashes, and the times of occurrence 
of the individual static pulses are, if not com¬ 
pletely random, at least highly irregular. The 
individual impulses are also irregular in ampli¬ 
tude. The overall spectrum of atmospheric 
static varies from day to day and from season 
to season, but static is usually most intense 
through the low and medium ranges of radio 
frequencies. The spectrum extends, however, 
into the high-frequency range and occasionally 
even beyond. Within the pass band of a radio 
receiver, which is very narrow relative to the 
overall range of the static spectrum, the aver¬ 
age distribution of static energy is essentially 
uniform. 

As a communication hazard, the most im¬ 
portant characteristics of a particular Sample 
of atmospheric static are its intensity level and 
its density. Density is simply a matter of pulse 
repetition frequency [PRF]. Although for a 
particular sample of static the number of pulses 
per second may vary within relatively wide 


limits, it is possible to think of the static as 
having an average PRF. For various samples, 
this average PRF may range from essentially 
zero to exceedingly high values, depending upon 
weather and propagation conditions. When the 
PRF is low and the pulses are widely spaced, 
the intermittent character of the noise is em¬ 
phasized. When, on the other hand, the PRF 
is high and the pulses closely spaced, static 
tends to take on the characteristics of fluctua¬ 
tion noise. a This effect is accentuated when such 
static is observed through a narrow-band re¬ 
ceiver. The individual pulses are broadened by 
passage through the narrow-band circuit and 
are forced to “pile up on each other’s heels,” 
thus giving rise to an irregularly fluctuating 
wave instead of an irregular sequence of spikes. 

The correspondence between this treatment 
of static characteristics and the “type” nomen¬ 
clature which has been widely used in previous 
discussions is fairly direct. Click static is weak 
with a very low PRF. Lightning and local 
thunderstorm static is intense with low or 
medium PRF. Hiss static is weak with a very 
high PRF and perhaps should be regarded as 
fluctuation rather than impulse noise. In order 
to describe the static frequently referred to as 
grinders, it is necessary to describe the “bursti- 
ness” of the static in terms, for example, of the 
percentage time occupied by bursts and their 
average frequency of recurrence. 

The diversity and inconstancy of natural 
atmospheric static make accurate measurement 
and rigorous specification imperative in tests 
which involve simulated static interference. 
For laboratory purposes methods of simulating 
atmospheric static were developed. If an audio 
simulation is desired, a satisfactory method 1 
consists of amplifying only the peaks of the 
ionization noise obtained from a gas-filled tube. 
The center-clipping or “peak-pass” circuit can 
be adjusted to control the density of the noise 

a Fluctuation noise is used as a generic term for noise 
characterized by highly irregular, aperiodic wave form. 
Shot noise, ionization noise, and thermal noise are 
examples. 


197 



198 


NOISE REDUCTION IN RADIO RECEIVERS 


peaks which are passed. To obtain radio-fre¬ 
quency [r-f] static, these audio static spikes 
can be boosted to high peak voltages by the 
action of a spark coil. 12a The circuit is shown 
schematically in Figure 1. When the voltage 
across the secondary of the coil reaches a 
critical value, the spark gap breaks down and 
there is a very brief surge of current through 
the 75-ohm resistor and through any external 
circuits in parallel with the resistor. The aver¬ 
age PRF can be controlled by adjusting the 
peak-pass circuit, and the PRF is indicated by 
a pulse-counting circuit at the output of a 
broad-band receiver. 

The general utility of such a static generator 
recommended the development of a more versa- 


SPARK COIL 



Figure 1. Schematic diagram of r-f static gen¬ 
erator. 

tile generator for use in calibrating noise 
meters and receivers, for acceptance tests of 
communication equipment, etc. Circuits which 
produce pulses of very short duration were 
designed and experimentally tested. A gen¬ 
erator of radio-frequency noise was built 8 
which provided steady or random PRF, the 
average PRF being adjustable over the range 
from a few cycles per second to approximately 
4,000 c. The amplitude of the pulses can be 
either constant or random with time. Three 
major types of pulse generators (hydrogen 
thyratron, argon thyratron, and hard tube 
types) were investigated, and the specific diffi¬ 
culties with each type propose problems for 
further development. 


13 2 RADIO VOICE COMMUNICATION 

The wave form of a static pulse suggests the 
possibility of limiting the peak amplitude of 
the pulse with a peak-clipping circuit. The 
ability of speech to withstand severe distortion 
of this type (see Section 7.1) would indicate 
that noise-peak limiters might operate without 


seriously affecting the intelligibility of the 
speech signal. 


13-2,1 Noise-Peak Limiters 

Extensive articulation tests were conducted 
to test the effects of peak-clipping in radio 
receivers. 3 The tests were made with the 
Wasmansdorff limiter in an ARB naval aircraft 
receiver. The limiter was inserted in the audio 
section of the receiver and was adjusted so 
that, in the absence of interfering noise, only 



1 3.16 10 316 100 316 


CARRIER VOLTAGE IN MICROVOLTS 

Figure 2. Effect of noise-peak limiter on articu¬ 
lation scores with static interference. 

the extreme peaks of the speech signal were 
affected. High-amplitude pulses were clipped off 
at this level, and the disturbing effect of the 
pulse was reduced. The limiter was arranged 
so that it could be turned on or off in successive 
tests. 

In tests conducted with moderately severe 
thunderstorm static as the only interfering 
noise, the limiter provided marked improve¬ 
ment in intelligibility, as shown in Figure 2. 
The increase in articulation score was as much 
as 15 percentage points. The improvement was 
restricted, of course, to the range of received 
signal levels in which the speech was too weak 
to override the interference. 

Tests in quiet and tests with the listeners in 























RADIO VOICE COMMUNICATION 


199 


airplane noise (no static) indicated that the 
amplitude distortion introduced by the limiter 
had little effect upon intelligibility when no 
interference was present in the received signal. 
Results obtained with both the announcer and 
the listeners in intense airplane noise showed 
that the limiter was less effective when noise 
picked up by the announcer’s microphone was 
present in the received signal. Even with the 
ambient noise interference, however, the 
limiter provided increased intelligibility with 
weak carriers in static. The maximum protec- 




Figure 3. Articulation results showing the com¬ 
bined effects of premodulation clipping and limit¬ 
ing the static peaks in the receiver. 


tion against static is obtained with noise-peak 
limiters when the microphone noise pickup is 
low. 

Although peak clipping has been most gen¬ 
erally used in the radio receiver, it is also pos¬ 
sible to clip the peaks of the speech signal 
prior to transmission. The advantage of pre¬ 
modulation clipping derives from the improved 
efficiency of modulation. The high-amplitude 
peaks of speech are removed at no loss in 
intelligibility, while the weak but important 
consonant sounds reach 100 per cent modula¬ 
tion. 

Tests using premodulation clipping 4 showed 
that the intelligibility of speech transmitted by 


amplitude modulation [AM] of a radio-fre¬ 
quency carrier could be considerably improved 
by a voltage-limiting device adjusted to permit 
no more than 100 per cent modulation of the 
carrier wave. The adjustment of the audio gain 
ahead of the limiter should produce 12 to 24 
db peak clipping of the speech signals, and a 
simple low-pass filter should be included after 
the voltage limiter to eliminate those audio 
frequencies above 5,000 c which result from 
peak clipping. 7 The transmission of amplitude- 
distorted speech, while not desirable for com¬ 
mercial broadcasting, is valuable for military 
communication where intelligibility is at a 
premium. Field tests supported the conclusions 
drawn from laboratory tests. 6 

Further tests explored the combined effects 
of clipping the peaks of the speech waves in 
an ATB transmitter and also limiting the static 
peaks with a Wasmansdorff noise limiter in an 
ARB receiver. 5 Typical results are shown in 
Figure 3 for two microphones and for two 
levels of premodulation clipping. The highest 
scores were obtained by clipping in both the 
transmitter and the receiver. Under the test 
conditions, the improvement over normal trans¬ 
mission amounted to 8 to 10 db. 

Since the noise-peak limiter is incorporated 
in the audio stages of the receiver, the noise 
pulses which it discriminates against are 
shaped in the r-f sections of the receiver. The 
response of a stagger-tuned band-pass network 
to a delta function (pulse of zero duration, 
infinite amplitude, and unit area) may be ap¬ 
proximated by two empirical relations. 10 The 
peak amplitude of the audio output pulse (or 
of the envelope of the r-f pulse) is 


where B n is the overall bandwidth measured at 
the half-power points, <£ is the slope of the 
selectivity curve plotted in terms of relative 
response in amplitude units versus frequency 
in cycles per second, and 4> is the slope of the 
selectivity curve plotted in terms of relative 
response in decibels versus frequency. Thus, if 
the slope of the selectivity characteristic is held 
constant, the amplitude of the output pulse 
increases linearly with bandwidth. Or, if band- 













200 


NOISE REDUCTION IN RADIO RECEIVERS 


width is held constant, the amplitude of the 
output pulse decreases as a hyperbolic function 
of the slope of the “skirts” of the selectivity 
curve. 

The duration of the output pulses as meas¬ 
ured at an amplitude 40 db below the peak is 

t = 1.80 = 0.154>. (2) 

Thus the duration at a particular relative 
amplitude level is independent of the receiver 
bandwidth and directly proportional to the 
slope of the skirts of the selectivity character¬ 
istic. 

The articulation results obtained in the pres¬ 
ence of impulse noise reflect the operation of 
these two factors, B n and <f>. As successively 
narrower pass bands with successively steeper 
skirts are employed, the amplitude and power 
of the output pulses decrease. At the same time, 
the duration of the output pulses increases. If 
no limiter is used, the reduction of amplitude 
due to the narrow pass band increases slightly 
the intelligibility of the speech, although the 
effect is not as important to intelligibility as 
might be supposed. When a noise-peak limiter 
is used, the controlling factor is the duration of 
the output pulses. If each output pulse is limited 
to the same amplitude, then each pulse essen¬ 
tially blanks out the speech signal during a 
short time interval. As the duration of the 
pulse is made longer, the disruptive effect on 
intelligibility is increased. When the output 
pulses are long enough to overlap, the noise 
assumes the characteristics of fluctuation noise, 
and the limiter becomes ineffective. Conse¬ 
quently, best results are obtained with noise- 
peak limiters if the output pulses are of short 
duration, i.e., if the selectivity characteristic 
has gradually sloping skirts. 

The effects of an amplitude selective device 
such as a noise-peak limiter can be summarized 
as follows: 

1. Amplitude limiters are ineffective against 
fluctuation noise because there is not, sta¬ 
tistically speaking, sufficient difference between 
the amplitude pattern of speech and the ampli¬ 
tude pattern of fluctuation noise to permit the 
device to favor one against the other. 

2. Against impulse noise, the amplitude 
limiter provides marked improvement in per¬ 


formance if it is incorporated in a receiver 
with appropriate selectivity characteristics. 

3. For optimal reception in the presence of 
impulse noise, the selectivity curve of the 
circuits preceding the limiter must have gradu¬ 
ally sloping skirts. 


1322 Static-Canceling Circuits 

Three noise-reducing circuits were developed 
and tested which operate on the principle that 
the impulse noise in adjacent radio frequencies 
can be used, by proper phase adjustment, to 
cancel out the impulse noise in the pass band 
of the receiver. The developmental work for 
two of these circuits, 2 which were intended for 
use with aircraft radio receiver AN/ARR-15, 
was done under Navy contract at Maguire 
Industries, Inc., and performance tests were 
conducted at the Psycho-Acoustic Laboratory. 
The third device was developed as a joint 
project by the Psycho-Acoustic Laboratory and 
Central Communications Research, Cruft Lab¬ 
oratory, Harvard University. 

Countermodulation 

The countermodulation circuit involves the 
use of an audio-noise voltage, provided by an 
auxiliary noise channel, to modulate the main 
signal in a special modulator incorporated into 
the intermediate frequency [i-f] section of the 
receiver. The main signal is, of course, the i-f 
carrier which has already been modulated by 
speech, and, in a sense, also by noise. The noise 
wave from the auxiliary channel is applied to 
the special modulator in proper amplitude and 
phase to counteract the noise component of the 
main signal. The result is a relatively noise- 
free i-f wave which can then be detected in 
the conventional manner. 

An assumption implicit in the countermodu-* 
lation scheme is that it is possible to obtain 
from the auxiliary noise channel a wave which 
contains more noise and less speech than does 
the signal in the main channel. This is accom¬ 
plished by detuning the noise channel slightly 
from the intermediate frequency of the re¬ 
ceiver, and by providing automatically adjusted 
bias to the noise detector or rectifier in order 



RADIO VOICE COMMUNICATION 


201 


to discriminate further against any low- 
amplitude signal component in the noise 
channel. 

A further assumption is that the wave form 
of the countermodulation voltage will be 
approximately the same as the noise envelope 
in the main i-f channel of the receiver. For 
certain types of noise, such as interfering car¬ 
riers or fluctuation noise with truly random 


RF SIGNAL-TO-STATIC RATIO IN OB 



Figure 4. Results of tests with simulated static 
at an intensity of 240 /^v and an average PRF of 
1000 pps. The countermodulation and W-pass 
circuits were developmental models, adjusted on 
the basis of preliminary tests to give optimal 
performance under the test conditions. The 
squelch circuit of the unmodified ARR-15 (No. 

6) was disabled for these tests. 

phase characteristics, this second assumption 
is not valid, but for the impulsive variety of 
interference it is well justified. Modulation 
envelopes due to impulsive interference tend 
to be highly similar in adjacent frequency 
bands, and, if the amplitude of the interference 
is high relative to that of the carrier, it is 
entirely possible to use noise from an auxiliary 
channel to cancel or to countermodulate noise 
in the main channel. 

Cancellation (W Pass) 

The same two assumptions are basic to the 
cancellation or W-pass circuit. This device uses 
cancellation in the audio section of the receiver 
to remove the noise. Three final i-f amplifiers 
and three detectors are used. Two of the ampli¬ 
fier-detector channels are detuned slightly, one 
to either side of the main intermediate fre¬ 
quency, in order to pass more noise than signal. 
The audio outputs of the three channels are 
mixed, with the noise detectors 180 degrees 
out of phase with the main detector. When the 


arrangement is properly adjusted, the noise 
voltages from the two noise channels cancel the 
noise component from the main channel, and 
relatively noise-free speech results. b 

Neither the W pass nor the countermodula¬ 
tion system should be expected to be effective 
against nonimpulsive interference. Even with 
impulse interference, the circuits cannot re¬ 
move all the noise from the desired signal. 

Performance Tests of Counter¬ 
modulation and Cancellation Circuits 

The countermodulation and W-pass circuits 
were built as adapters, arranged to replace the 
circuits of the AN/ARR-15 receiver. 12 The 

RF SIGNAL- TO- NOISE RATIO IN DB 



Figure 5. Curves showing the performance of 
the test and reference receivers in the presence 
of fluctuation noise. 


method of testing involved the use of an ARB 
receiver as a reference or control to compare 
with the experimental receivers. In order to 
study the action of the noise-reducing circuits 
under a variety of conditions, articulation tests 
were conducted with several types of interfer¬ 
ence: irregular static pulses, regular pulses 
similar to the interference from a radar modu¬ 
lator unit, fluctuation noise, and spurious radio 
signals in the form of interfering carriers. 

b Since two noise channels are used, it is convenient 
to use the term W pass to designate this system. This 
term is proposed because the two auxiliary channels, 
one on either side of the main channel, give to the com¬ 
posite selectivity curve of the receiver a form suggestive 
of the letter W. 









202 


NOISE REDUCTION IN RADIO RECEIVERS 


Results of the tests with impulse noise are 
summarized in Figure 4. The ARB reference 
receiver is slightly superior to the ARR-15. As 
indicated by the figure, over 50 per cent word 
articulation was obtained with the noise-cancel¬ 
ing circuits at a carrier intensity of 3.2 pv 
(—37 db r-f signal-to-static ratio), whereas 
the unmodified receiver required almost 180 
pv (—2 db r-f signal-to-static ratio). This con¬ 
stitutes a 35-db improvement in performance 


AUDIO MODULATING 
SIGNAL 


■ transmitter 


CARRIER,f, 


TWO-CARRIER SIGNAL 


CARRIER,f 2 



AUDIO 

OUTPUT 


DISCRIMINATOR 





OUTPUT PROM 
DISCRIMINATOR 
SECTION GIVING 
PEAK AT f, 


JULA 


INPUT VERY 
SHORT IMPULSES 
CONTAINING 
BOTH f, AND f 2 


OUTPUT FROM 
DISCRIMINATOR 
SECTION GIVING 
PEAK AT t 2 



COMPLETE 

DISCRIMINATOR 

OUTPUT 


Figure 6. Operation of two-carrier transmission 
system. 


due to the countermodulation or W-pass 
circuits. 

No attempt was made to quantify the im¬ 
provement provided against regular-pulse in¬ 
terference because there was difficulty in 
providing enough interference to tax the capa¬ 
bilities of the static-canceling circuits. It was 
apparent, however, that marked improvement 
in performance could be obtained. 

The results of tests in the presence of fluctua¬ 
tion noise showed that there are essentially no 
differences in performance between the modi¬ 
fied and unmodified receivers. These results are 
indicated graphically in Figure 5. 

Tests with unmodulated carriers and with 
frequency-modulated carriers used as inter¬ 
ference likewise showed no significant differ¬ 
ence between the ARR-15 with noise-reducing 
circuits and the reference receiver. 

The static-canceling circuits are, therefore, 


very effective against impulsive types of inter¬ 
ference but have no effect against the other 
types. Since many of the serious sources of 
interference with military radio communica¬ 
tions are of the impulsive type, the counter¬ 
modulation and W-pass circuits hold much 
promise for future development and applica¬ 
tion. 

Two-Carrier System 

This radio-communication system makes use 
of two carrier frequencies with a special type 
of amplitude modulation. 9 The two carriers are 
separated in frequency by a few kilocycles. One 
carrier is modulated in amplitude by the posi¬ 
tive portion only of the audio modulating 
signal, the second by its negative portion only. 
In the absence of an audio modulating signal 
neither carrier is present. At the receiver a 
sort of frequency discriminator is used as a 
detector, with the positive and negative peaks 
of the discriminator response curve coinciding 
with the carrier frequencies. The action of this 
circuit is such that the original audio modulat¬ 
ing signal is obtained from the two modulated 
carriers. Electric interference of a sharply 
peaked character affects the discriminator so 
that the noise is balanced out in the output. 
A simple limiting circuit follows the discrimi¬ 
nator and removes any noise peaks which may 
remain because of imperfect balance. The re¬ 
sult is a great improvement in effective signal- 
to-static ratio. 

A diagram illustrating the operation of the 
two-carrier system is shown in Figure 6. 

One of the major advantages of this system 
of noise reduction stems from the fact that, in 
order to transmit sinusoidal audio signals, just 
one-third the amount of r-f power is required as 
would be needed by the conventional AM system 
employing a single carrier. The system behaves 
in this respect much like the suppressed carrier 
transmission system, but without the necessity 
of having to reinsert the carrier at the receiver. 

The major disadvantage of a two-carrier 
system is the broad channel required for trans¬ 
mission. The carriers must be separated suffi¬ 
ciently to reduce side-frequency interference 
and to make the beat frequency between the 
two carriers inaudible. On the other hand, 


















RADIO VOICE COMMUNICATION 


203 


the smaller the separation of the two car¬ 
riers the better is the noise cancellation pro¬ 
vided by the discriminator. Probably 15 kc is 
near the optimum separation. 


used to pick up a single carrier whose frequency 
corresponds to that of one of the two carriers 
for which the receiver is adjusted, and which 
has normal amplitude modulation. The inter- 



Consideration of the two circuits will show ference reduction obtained with this single 
that the two-carrier receiver is very similar in carrier is almost exactly the same as that 
principle to the countermodulation circuit dis- obtained when the receiver picks up the two 
cussed above. The two-carrier receiver may be carriers with their special type of amplitude 





















204 


NOISE REDUCTION IN RADIO RECEIVERS 


modulation. Thus, the noise-reducing capacity 
of the system is inherent in the receiver alone. 
In either case, the receiver contributes about 
35 db improvement over normal operation. 


133 RADIO-RANGE SIGNALS 

Since 1929, the principal radio aid to aircraft 
navigation has been the low-frequency radio 
range. In 1945 there were approximately 200 
range stations, operating on frequencies be¬ 
tween 200 and 400 kc, and providing a network 
of radio airway markers covering the United 
States. 

The essential features of navigating by radio 
range are illustrated in the schematic repre¬ 
sentation of Figure 7. The range transmitter 
is located at the right-hand side of the figure, 
and radiates dot-dash signals (code letter A) 
with maximum intensity along the upper 
boundary of the sector, and dash-dot signals 
(code letter N) with maximum intensity along 
the lower boundary of the sector. Field strength 
is represented by density of shading. For pur¬ 
poses of illustration, a number of A’s and N’s 
are shown simultaneously in the range area. 

The on-course line represents the line at 
which the A and the N signals have the same 
field strength, and the two signals interlock to 
form a steady tone of 1,020 c in the operator’s 
headphones. Near the left-hand side of the 
figure the signals are weak and, therefore, diffi¬ 
cult to make out against background noise. 
Near the on-course line the differential between 
the A and the N is low, and it is difficult to 
discern which signal predominates. An air¬ 
plane approaching the transmitter along the 
line marked “path of plane” would, then, fly 
into the N zone until it reached a point (1) at 
which the field strength was high enough for 
adequate reception and the differential between 
N and A was sufficiently marked for the pilot 
or the navigator to make out the dash-dot of 
the N. Discovering that the N was growing 
progressively stronger, the pilot would correct 
his course and fly toward point (2). At point 
(2) he would again correct his course, etc. In 
this way he would follow or bracket the path 
along which he could just distinguish the N. 


This is sometimes called the “edge” of the beam, 
and is not, of course, sharply defined. “Flying 
the edge” tends to prevent collisions with air¬ 
craft flying in the opposite direction along the 
same course. 

Of the two systems used to transmit range 
signals, one is the so-called “simultaneous” 
range which provides both range and voice 
signals on a single channel; the other is a 
simpler system which provides range signals 
only. The simultaneous range, also called five- 
tower range, is the more widely used. The 
simpler system employs two directional anten¬ 
nas, usually crossed loops. 

The simulation, for laboratory tests, of radio¬ 
range signals must consider several general 
factors which govern the performance of radio¬ 
range systems, one of the most important of 
which is the human factor. The ability of the 
ear to distinguish small variations in the in¬ 
tensity of a tone, and the effects of background 
noise upon the accuracy of discrimination were 
discussed in Section 3.5. Perhaps the most 
important physical variables are the signal-to- 
static ratio, the A-to-N ratio, the density of the 
static, and the characteristics of the range re¬ 
ceiver. Some of these factors, such as the dif¬ 
ferential sensitivity of the ear or the character¬ 
istics of the audio sections of the receiver, can 
be tested with simple audio equipment. 11 For 
a complete picture, however, it was necessary 
to use r-f test equipment. For most tests 
R-23/ARC-5 and R-23A/ARC-5 receivers were 
used. 


13-31 Radio-Frequency Signal-to-Static 
Ratio and A-to-N Ratio 

As a first step in studying the performance 
of a radio-range system, it is important to 
determine the ways in which the distinguisha- 
bility of the received A-to-N signals depends 
upon the r-f signal-to-static ratio and upon the 
A-to-N ratio. Tests were conducted with a 
standard range receiver and with simulated 
five-tower range signals. The r-f static had an 
average PRF of 1,000 and an intensity of 
22 (xv. 13 

Results of these tests are shown in Figure 



RADIO-RANGE SIGNALS 


205 


8, where the ratio necessary to distinguish A 
from N is plotted in decibels as a function of 
the r-f signal-to-static ratio. The curves, which 
are very similar to those obtained with audio 
equipment (see Section 3.5), illustrate the 
necessity of having a large A-to-iV differential 
when the static conditions are unfavorable. 
Thus the range is essentially many degrees 



Figure 8. Curves showing the paired values of 
A-N ratio and r-f signal-to-static ratio required 
for 60-, 75-, and 90-per cent correct reception of 
five-tower range signals. 


wider at a distance from the range transmitter 
than it is near the transmitter where the signal- 
to-static ratio is more favorable. 


13 ’ 3 2 Static Density 

The curves of Figure 9 show how markedly 
the actual interference value of the static is 
dependent upon the static density. High PRF 
static interferes seriously with reception, and 
a relatively high signal-to-static ratio is re¬ 
quired for any particular level of performance. 

As indicated by the relative positions of the 
curves, the interference value of the static at 
first rises rapidly with increasing PRF but, 
with further increase, a “saturation” point is 
approached. This saturation point is a function 
of the bandwidth of the range receiver and 
corresponds to the point at which the audio 
static begins to lose its spike-like wave form 
and to take on the appearance of fluctuation 
noise. For conventional range receivers, the 


bandwidth is 2 to 3 kc, and the critical PRF 
is approximately 3,000 pps. 


13 ' 3 ' 3 Beam Filters 

The simplest and most widely used noise- 
reducing circuit for radio-range receivers is 
the “beam filter.” The function of the filter is 
simply to restrict the signal delivered to the 
operator’s headphones to a narrow range in 
the region of 1,000 c, thereby eliminating a 
large amount of the static and little or none 
of the desired signal. lla 

Radio-frequency tests were conducted with 
a beam filter of the type (NAF 68304) used by 
the Navy during the war. 13a Results indicated 
that the filter is more effective against fluctua¬ 
tion-type noise than it is against low PRF static. 
When the resonant circuit of the filter is ener¬ 
gized by a static pulse, a damped oscillation or 
“ringing” results at the resonant frequency. 
The tendency of the filter to “ring” in the pres¬ 
ence of intermittent static may nullify any 


A-N RATIO 3.5 DB 



RF SIGNAL-TO-STATIC RATIO IN DB 


Figure 9. Curves showing the relation between 
static density (the parameter) and range per¬ 
formance. 

possible advantage it may provide, because the 
ringing sound of the static is highly similar to 
the sound of the range signal itself. It would 
be dangerous, therefore, to quote a particular 
figure as representative of the improvement in 
range performance provided by a beam filter. 
Informal observations suggest that, in a con¬ 
ventional range receiver and under certain 





206 


NOISE REDUCTION IN RADIO RECEIVERS 


noise conditions, the beam filter should make 
it possible to fly a narrow beam with about 10 
db less r-f signal strength. 


13-3-4 Noise Limiters 

Series-diode noise-peak limiters were in¬ 
corporated in two standard range receivers. 
Figure 10 shows the results of tests conducted 
with a relatively high A-to-iV ratio (3.5 db). 


A-N RATIO 3.5 DB 



DENSITY OF STATIC IN PPS 


Figure 10 . Improvement in radio-range per¬ 
formance provided by a noise limiter in the range 
receiver. The improvement was measured in 
terms of the reduction in the r-f signal-to-static 
ratio which was required for 75-per cent correct 
reception. 

The improvement provided by the limiter is 
plotted as a function of the density of the static. 
With PRF’s of 100 and 500 pps, the limiter 
made it possible for the listeners to record 75 
per cent of the range signals correctly with 5 
to 7 db less signal intensity than was required 
without the limiter. However, the limiter pro¬ 
vided no improvement at all with static at 
3,000 pps. 

Thus the conditions under which the noise 
limiter showed its best performance were the 
opposite of those under which the beam filter 
had proved effective. 

Inasmuch as the beam filter and the noise 
limiter were effective under different condi¬ 
tions, it appeared that they might be used with 
advantage in combination. It was found, how¬ 
ever, that they did not work well together. In 
clipping the stronger component (for example, 
the A) of the range signal more severely than 
the weaker N, the limiter introduced stronger 
harmonics into the A than into the N. When no 
beam filter was used, the A had a distinctive 
quality which helped the listeners to distinguish 


it from the N. When the filter was introduced 
between the output terminals of the receiver 
and the headphones, however, the distinctive 
harmonics were attenuated. Thus one of the 
advantages derived from the use of the noise 
limiter was lost when the filter was added. 

It was thought that this incompatibility 
might be reduced by the devising of a limiter 
which would clip weak signals more severely 
than strong ones. Such an “expander-limiter” 
was developed, and proved more effective than 
the usual type of limiter, 13b but even its per¬ 
formance was not all that was expected on the 
basis of experience with noise limiters in com¬ 
munication receivers. 

Tests with receivers of different bandwidths 
indicated that the high selectivity of range 
receivers (dictated by the fact that the width 
of range channels is narrow) is the principal 
factor reducing the effectiveness of limiters. 
This effect may be explained in terms of the 
pulse-transmission characteristics of band-pass 
circuits (see Section 13.2.1). 


13-3 ' 5 Static-Canceling Circuit 

A static-canceling circuit similar to those 
described in Section 13.2.2 was tested in the 
presence of simulated thunderstorm static, and 
its performance was far better than that of 
the standard range receivers previously dis¬ 
cussed. In order to tax the static-canceling 
receiver it was necessary to use static inten¬ 
sities above 1,000 pv, and even under these 
conditions the low PRF static provided less 
interference than the background noise of the 
receiver. With the static-canceling it was pos¬ 
sible to “navigate” with signal-to-static ratios 
about 50 db lower than would have been re¬ 
quired with a standard range receiver. When 
fluctuation noise was used, however, the per¬ 
formance was the same with the canceling 
circuit as without it. 

These results for static of reasonably low 
PRF are suggestive of the effectiveness of the 
canceling principle. It is probable that, in the 
presence of low PRF static, equally good re¬ 
sults might be obtained with narrow-band 
circuits more suitable to radio-range applica- 





RADIO-RANGE SIGNALS 


207 


tions. Further evaluation must rest upon field 
tests or upon operational experience. 


Other Radio-Range Devices 

More or less as by-products of the study of 
radio-range receivers, two simple devices were 
developed which, when plugged into the output 
of a range receiver, allow the pilot to listen 
with greater comfort to the range signal. One 
of these, the radio-range signal expander, llb 
filters out the static and expands the voltage 
differential between the A and N signals. At 
reasonably large signal-to-noise ratios the 
range reception is considerably improved by 
this device. 

The second circuit, the pitch modulator, 110 
converts the usual intensity variations into 
pitch variations, with the result that static 
merely raises the general pitch level or, at 
worst, causes a ragged pitch signal instead of 
noise. Thus the pilot has to distinguish the 
relative variation of pitch, but does not have 
to listen to the loud noise introduced by static 
in the ordinary radio receiver signal. This 
device is also very effective with favorable 
signal-to-noise ratios. 


13.3.7 Reception of Voice Signals 

Inasmuch as one of the important functions 
of the “simultaneous” range is the transmission 
of weather information by means of voice sig¬ 
nals, it is important to consider the effects of 
the various noise-reducing circuits upon the 
reception of speech. One of the major diffi¬ 
culties lies in the high selectivity and poor audio 
frequency response of the receiver itself. The 
pass band of a conventional range receiver is 
too narrow for fully effective voice communi¬ 
cation. 

When voice signals are being received, a 
narrow band-rejection filter is customarily in¬ 
serted to attenuate the range signals. Tests 


have shown that the actual masking caused by 
the 1,020-c tone is negligible (see Section 8.2), 
but the rejection filter may serve to reduce 
annoyance. However, the conventional filter 
eliminates a band of speech frequencies 300 to 
400 c wide, and produces a decrement in intelli¬ 
gibility of approximately 6 per cent. The results 
of articulation tests conducted with audio 
equipment are shown in Figure 11, where the 
per cent word articulation is plotted as a func¬ 
tion of the width of the rejected band for 
different speech-to-noise ratios. These data in¬ 
dicate that the filter, if used, should have the 
narrowest possible bandwidth. 

Noise-peak limiters and static-canceling sys¬ 
tems have been shown in preceding sections 
to be about equally effective for speech and 
for range signals. The limiter is somewhat 
handicapped, however, by the narrow pass band 
of the typical range receiver. 

On the basis of these results, two avenues 
for future development seem feasible. First, the 



Figure 11. Per cent articulation as a function 
of the width of the suppression band. 


possibility of using i-f noise limiters instead 
of audio limiters in range receivers should be 
explored. Locating the limiter ahead of some of 
the selective circuits should provide the advan¬ 
tage of wide-band limiting. Second, a narrow- 
band, static-canceling circuit should be de¬ 
veloped and subjected to field tests under severe 
conditions of noise. Success in these two proj¬ 
ects would ensure considerable improvement in 
the performance of five-tower range equipment. 




Chapter 14 

SELECTING AND TRAINING PERSONNEL 


N O COMMUNICATION equipments are more 
effective than the people who use them. 
The finest equipment built cannot establish 
voice communication between a tongue-tied 
talker and a deafened listener. Maximum trans¬ 
fer of information is only achieved when the 
personnel as well as the equipment are the 
best available. 

In order to obtain the most efficient personnel, 
a careful program of selection and training 
must be devised. Differences between indi¬ 
viduals in their abilities to hear and to be heard 
in intense noises are surprisingly large, and 
the selection of the better talkers and listeners 
depends upon the accurate estimation of these 
individual differences. Also, the performance 
of both listeners and speakers improves after 
even a brief period of practice and instruction. 
The experience of the Psycho-Acoustic Labora¬ 
tory in evaluating and training military talkers 
and listeners is summarized in this chapter. 


141 SELECTION OF TALKERS 

In the development of adequate articulation 
testing methods (see Chapter 5), it quickly 
became apparent that individual differences 
were considerable when apparently “normal” 
talkers spoke in the presence of intense noise. 
Analyses of such individual differences 1 sug¬ 
gested the possibility of providing relative esti¬ 
mates of speaking ability which would be valid 
for the conditions of actual military operation. 
As a result of these early and more or less 
incidental observations a separate project was 
undertaken to explore the problem in greater 
detail. 

In order to obtain an adequate sample of 
talkers, both word and sentence articulation 
scores were obtained for three groups of talkers 
numbering 47 men in all: (1) group R, 21 
talkers chosen at random from an undergradu¬ 
ate college population; (2) group F, 13 talkers 
chosen from a group of 270 enlisted men by a 
competent speech correctionist as exhibiting 


defective speech characteristics; and (3) group 
P, 13 talkers chosen at random from the re¬ 
maining 257 enlisted men. A series of experi¬ 
ments were conducted, with these 47 talkers, 
which had as an object to determine (1) the 
value of subjective ratings of talkers as an 
index of their measured intelligibility, and 
(2) the basic factors underlying the individual 
differences and determining the intelligibility 
of the speech. 


1411 Subjective Ratings of the 

Intelligibility of Talkers in Noise 

A number of articulation tests were con¬ 
ducted to provide intelligibility scores for 
talkers in noise, and analyses were made of the 
relations between the articulation scores and 
the scores derived from three types of subjec¬ 
tive ratings of the same talkers: (1) pre¬ 
liminary ratings based on the standard speech 
interview for Harvard freshmen made by the 
instructors in an English course (Navy V-12), 
(2) ratings made during articulation tests by 
the 7 to 12 women composing the listening 
crew, and (3) ratings made both before and 
during the course of the articulation tests by 
two members of the Psycho-Acoustic Labora¬ 
tory. 48 

On the basis of the preliminary ratings from 
the standard speech interview, a group of men 
(group F) was singled out as exhibiting espe¬ 
cially faulty or undesirable speech traits. As 
a control, an equivalent group (group P) was 
chosen at random from the other members of 
the course who had satisfactorily passed the 
speech interview. Both groups were tested for 
their intelligibility in noise, with words and 
complete sentences used as the speech material. 
For both types of material, the randomly 
selected group (group P) achieved a higher 
mean score. Although the differences between 
the means were significant in a statistical 
sense, the two groups were not nearly so dif¬ 
ferent as might have been expected. On both 


208 


SELECTION OF TALKERS 


209 


words and sentences, for example, some of the 
talkers who had been considered faulty by 
normal standards of judgment were among the 
more intelligible, and some of the randomly 
selected were among the less intelligible of the 
entire body of men tested. The overlap is shown 


a part of the criteria for superior speech in the 
absence of noise seems applicable in determin¬ 
ing this type of intelligibility. 

During the testing of these talkers, each 
member of the listening crew was asked to 
estimate the number of words which she had 


e 75 


ES ° 

o 


MW OB 

x o 


X GROUP P (PASSED) 
O GROUP F (FAILED) 


35 40 45 50 55 60 65 70 75 

PERCENT WORD INTELLIGIBILITY 

Figure 1. Word intelligibility vs sentence intelligibility for two groups of talkers. Group P was chosen 
randomly from those who had passed, and group F consisted of those who had failed a college check-list 
speech interview. 


in Figure 1 for both word and sentence ma¬ 
terials. 

It was concluded, therefore, that ratings 
made on the basis of a standard speech inter¬ 
view appropriate to the requirements of a col¬ 
lege course in speech are of limited validity for 
the peculiar situation of a military talker who 
must make himself intelligible in noise. Only 


identified correctly. The listeners were never 
told the correct forms of the words or the 
sentences read, or the actual intelligibility 
scores achieved. It turned out that on both 
words and sentences the mean estimated scores 
showed high correlations (0.83 to 0.99) with 
the mean intelligibility scores. The magnitude 
of the correlation was maintained for various 




210 


SELECTING AND TRAINING PERSONNEL 


groups of military and civilian talkers and for 
various types of microphones and interfering 
noises. After a period of practice, although the 
listeners had no access to information about the 
accuracy of their estimates, the discrepancy 
between estimated scores and intelligibility 
scores decreased until the two approximated 
closely. 

In addition to these ratings made by the 
listening crew, two of the experimenters rated 
the intelligibility of a number of talkers on a 
seven-point scale. Ratings made in a prelim¬ 
inary interview under quiet conditions and also 
with noise introduced electrically into the ear¬ 
phones of talker and judges yielded a moderate 
correlation with later intelligibility scores. 
Other ratings by the same observers made dur¬ 
ing the articulation tests themselves showed a 
consistently higher correlation with the articu¬ 
lation score. 

It would seem, therefore, that trained judges 
who emphasize factors found to influence the 
intelligibility of a voice can probably make 
valid ratings of a talker’s capacity to make 
himself intelligible in noise. Should it prove 
in some cases inconvenient to obtain the serv¬ 
ices of experienced speech analysts, the indica¬ 
tions are that valid estimates of the intelli¬ 
gibility of talkers could also be obtained from 
the average results of a small group of novice 
raters. These ratings would be based on a form 
of articulation test modified so as to require 
only a few minutes of testing for each talker. 
The results of the experiment indicate that the 
amount of success to be expected from ratings 
made at a naive level without specific training 
in the task or knowledge of the results is fairly 
high. The efficiency of ratings could be im¬ 
proved greatly by special training. 


14-1-2 Factors Related to the Intelligibility 
of Talkers in Noise 

The second aspect of the study of individual 
differences among talkers was directed at the 
problem of understanding what it is about 
human voices that makes for clarity and intelli¬ 
gibility in the presence of noise. 4 

In order to study and analyze a group of 


voices whose relative effectiveness under the 
stress of noise was known, the 47 talkers were 
given both word and sentence intelligibility 
tests. The intelligibility scores thus obtained 
were used as “criterion scores” against which 
were correlated a variety of analytical measures 
made on the individual voices. Since the pro¬ 
cedures employed require repeated measure¬ 
ments on each voice, there were recorded on 
phonograph disks several samples of both the 
loud and the conversational speech of each 
talker. From these samples measurements were 
made of the following physical attributes: (1) 
the overall intensity level of the voice, (2) the 
distribution (spectrum) of speech energy in 
the frequency range from 80 to 4,000 c, (3) the 
pitch of the voice (fundamental frequency), 
and (4) the “peak factor” of the voice (ratio 
of peak to effective voltage: effective voltage 
was read on a VU meter, peak voltage on an 
oscilloscope). 

Also, in order to evaluate some speech attrib¬ 
utes that are not amenable to physical meas¬ 
urement, five judges made systematic subjective 
ratings of: (1) consonant strength, (2) con¬ 
sonant precision, (3) appropriateness of rate 
of speaking, (4) duration of words, (5) steadi¬ 
ness of rate of speaking, (6) steadiness of level 
and emphasis, (7) noise penetrating quality, 
(8) deviation from general American dialect, 
and (9) overall intelligibility. 

Analysis of the interrelations among the 
physical measurements, the subjective ratings, 
and the criterion intelligibility scores led to 
several conclusions. 

When talkers are instructed to speak in “the 
loudest voice possible without strain” the in¬ 
tensity of their voices shows a low positive 
correlation (about 0.28) with the criterion in¬ 
telligibility scores. When talkers possessing 
defective speech characteristics or noticeable 
local dialects are eliminated, however, these 
correlations rise to 0.45. The loudness which 
the talkers choose when instructed to assume 
a conversational level shows no correlation with 
the criterion intelligibility scores. 

Various indices of the shapes of the indi¬ 
vidual voice spectra were devised, but no sig¬ 
nificant correlations were found between these 
indices and the intelligibility scores. 



SELECTION OF TALKERS 


211 


The pitch, or fundamental frequency of 
the voice, was found to vary directly with its 
intensity level. Any relation between pitch and 
intelligibility, however, was due principally to 
their common relation with intensity. When 
differences in intensity were held constant by 
means of partial correlation, pitch showed a 
slight negative correlation with intelligibility. 
Given two voices of equal intensity, the one 
with the lower pitch was apt to be the more 
intelligible. 

Peak factors varied widely between talkers, 
but no correlation was observed with word 
articulation scores. A slight correlation was 
observed with sentence intelligibility. 

Of the aspects of the talkers’ voices which 
were rated, excellent agreement among the 
judges was obtained for ratings of overall in¬ 
telligibility, noise penetrating quality, conso¬ 
nant precision, dialect, and consonant strength. 

The average estimated intelligibility ratings 
on different types of speech materials corre¬ 
lated as high as 0.73 with the actual intelligi¬ 
bility scores. By multiple correlation, it was 
found that the overall ratings of intelligibility 
were dependent principally upon the variables 
of noise penetrating quality, consonant strength, 
consonant precision, and dialect. 

The subjective ratings which were most 
closely related to the intelligibility scores were 
those for consonant strength and consonant 
precision. Absence of dialect was found to be 
more important for sentence intelligibility than 
for word intelligibility. 

Contrary to expectations, estimates of the 
rate of talking proved of little importance in 
predicting the criterion intelligibility scores. 
However, since talker training courses often 
embody recommendations for the optimal num¬ 
ber of words per minute, it is of interest to 
note what rate the judges considered optimal. 
At the conversational level, the mean number 
of words read per minute was 164, while the 
judges considered that 130 to 140 words per 
minute was optimal. At the loud voice level, the 
talkers tended to slow their reading rate con¬ 
siderably so that the average number of words 
per minute was 141. The judges still considered 
this too rapid a rate and their judgments indi¬ 
cate that 120 words per minute would be op¬ 


timal when speaking in a loud voice. In Figure 
2, mean ratings assigned by the judges are 
presented for the various rates used by the 



Figure 2. Ratings assigned by 5 judges are 
shown for the various speech rates used by the 
47 talkers at both the loud and the conversational 
voice levels. 


talkers. The ratings have been transformed to 
a common scale extending from 0 to 1. The 
indication is that it is possible to read too 



RATE IN WORDS PER MINUTE 
SPOKEN IN A LOUD VOICP 

Figure 3. Showing the average relationship be¬ 
tween the rate of speaking and sentence in¬ 
telligibility. 

slowly but that in the opinion of the judges it 
was not a common fault. 

When the actual number of words per 
minute spoken by the talkers are compared 
with their intelligibility scores, the opinion of 








212 


SELECTING AND TRAINING PERSONNEL 


the judges seems to be substantiated. There is 
a low negative correlation between intelligibil¬ 
ity and speech rate, with a great deal of scatter, 
but if the intelligibility scores of all the talkers 
reading at rates between 100 and 120 words 
per minute, 120 and 140, etc., are averaged, 
the function presented in Figure 3 is obtained. 
Slightly better scores were made by the slowest 
talkers. There is little loss in intelligibility, 
apparently, until a rate of 160 to 180 words per 
minute is reached. 

The practical import of these investigations 
for the rating and training of talkers is that 
intelligibility in noise can be attributed to no 
single voice dimension but is a complex re¬ 
sultant of the talkers’ ability to speak loudly, 
to articulate speech sounds correctly and 
strongly, and to speak in a loud, resonant voice 
which seems to have the attribute of audibility 
through noise. The aspects of pitch, rate, dura¬ 
tion, and steadiness were found to add little to 
the accuracy with which the intelligibility of 
the voices could be predicted on the basis of the 
other variables. 


14 2 SELECTION OF LISTENERS 

In many ways the problem of devising an 
adequate method of selecting the better listen¬ 
ers is simpler than the problem of selecting 
talkers. In any adequate program of talker 
selection some time must be spent with each 
individual talker, and even though a speech 
rating interview may be considerably abbrevi¬ 
ated, adequate ratings of a large group of talk¬ 
ers is a time-consuming process. The selection 
of listeners, however, can go forward much 
more rapidly, since as many listeners can be 
tested simultaneously as can be seated together. 
Also, the criterion for listening ability may be 
more objectively determined than for speaking 
ability, since by the use of phonographically 
recorded speech samples, listening tests can be 
prepared and standardized for general use. 

The ideal test of listening ability should meet 
several criteria. First and probably most impor¬ 
tant is the problem of validity for the military 
situation. The fact that individual differences in 
ability to hear through a variety of different 


types of interfering noises are highly correlated 
greatly simplifies this problem. As a result, it is 
possible to develop a test using one particular 
type of noise interference and to feel confident 
that this test will also yield a valid indication of 
listening ability in most other types of noise. 
The generality of this statement should be con¬ 
firmed, however, for any unusual types of inter¬ 
ference. It was found, for example, that the 
ability to hear speech through a tonal type of 
interference did not correlate satisfactorily 
with ability to hear through a random noise. 
The greater annoyance and unpleasantness pro¬ 
duced by the tonal type of signal probably 
introduced psychological factors not sampled 
by the random noise test. Consequently, a sep¬ 
arate test had to be developed for this specific 
application. 7 - 8 

It is also important to ascertain that differ¬ 
ences in intelligence do not affect the scores. 
If, for example, test words are used which lie 
beyond the vocabulary of the average person 
for whom the test is intended, items will be 
missed because they are not understood, even 
though they are clearly heard. For most tests, 
therefore, it is desirable to minimize the effects 
of intelligence on the test score. In many mili¬ 
tary situations, however, it is desirable to select 
listeners not only for their ability to hear 
speech in noise, but also on the basis of that 
aspect of intelligence which enables them to 
repeat what has been heard. Consequently, a 
phonographically recorded test of the listener’s 
memory span for digits was developed. 3 While 
this is not a test of listening ability in the re¬ 
stricted sense, it may well prove useful in a 
battery of tests designed to select listeners for 
specific jobs. 

As a second consideration, the listening tests 
must be reliable. If a test is given twice to the 
same group, the listeners should be rated ap¬ 
proximately in the same way by both admin¬ 
istrations of the test. Important in this respect 
is the level of difficulty of the test. In order to 
provide the maximum discrimination between 
different listeners, it is necessary to eliminate 
from the test those items which no one misses, 
and also those items which no one hears cor¬ 
rectly. Such items are dead wood and do not 
discriminate one listener from another. Conse- 



SELECTION OF LISTENERS 


213 


quently, experimentation is necessary in order 
to obtain a signal-to-noise ratio which provides 
a satisfactory level of difficulty and of test 
reliability. 

In order to provide valid and reliable tests 
of listening ability, different types of speech 
material were recorded against a background 
of masking noise. 5 These tests, available on 
phonograph disks and suitable for group ad¬ 
ministration, were designed to be most efficient 
when administered over resonant earphones 
(TH-37 or R-14), but they have also proved to 
be of adequate reliability when played over a 
loudspeaker or over nonresonant earphones 
(ANB-H-1 or ANB-H-1A). 

The three listening tests of value for listener 
selection are: 


1. Words in Noise (Write-down Type) 


Name: 


Items: 


Scoring: 

Administration 

time: 


Reliability: 


Auditory Test No. 2 (previously 
issued as Series L * Type W). 
200 discrete words spoken to the 
listeners who write them down. 
By hand. 

30 minutes for the long form 
(items 1 to 200) ; 20 minutes for 
the short form (either items 1 
to 100 or 101 to 200). 

For 200 items, 0.94 (using 
TH-37 earphones). 


2 . 


3. 


Sentences in Noise 
N ame: 

Items: 


Scoring: 
Administration 
time: 

Reliability: 
Sentences in Noise 
Name: 

Items: 


Scoring: 

Administration 

time: 

Reliability: 


(Instructional Diagram Type) 
Auditory Test No. 4. 

100 commands, for each of which 
four drawings or diagrams are 
given. The listener answers by 
marking one of the diagrams. 
By hand. 

25 minutes. 

0.93 (using TH-37 earphones). 
(Multiple-Choice Type) 
Auditory Test No. 8. 

100 sentences, consisting of 
questions, commands, and in¬ 
complete statements, for each of 
which four alternative words or 
numerals are given. The lis¬ 
teners answer by underlining 
the appropriate response. 

By IBM machine or hand stencil. 

25 minutes. 

0.90 (using TH-37 earphones). 


Each of these three tests appears to be a 
satisfactory measure of the ability to listen in 
noise. The choice between them should be based 
on the relevance of the speech material for the 


military situation, and the practical consider¬ 
ations of administration and scoring. Each of 
the tests takes about 25 minutes to administer 
and has been given successfully to as many as 
110 men at one time (see Figure 4). 

Although tests were developed using both 
discrete words and sentence material, there 
seems to be little difference between the ability 




Figure 4. Showing two groups of subjects 
taking listening tests. 


to interpret meaningful sentences and the abil¬ 
ity to hear discrete words in noise. Likewise, 
the results obtained seemed to be independent 
of whether the observer is asked to write down 
what he hears or whether he is asked to choose 
the correct item from a number of different 
items. Further, since the score on the test de¬ 
pends primarily upon the signal-to-noise ratio 
rather than upon the overall level employed, the 









214 


SELECTING AND TRAINING PERSONNEL 


scores on the tests are not significantly affected 
either by the variations in sensitivity among 
earphones of the same basic type or by the 
seating position of the listeners relative to a 
loudspeaker. For any one test, however, the 
listener’s score depends upon the type of trans¬ 
ducers and playback equipment used. There¬ 
fore, unless similar equipment is used at all 
testing centers, only the relative standings of 
listeners at different centers and not their ab¬ 
solute scores may be considered comparable. 

A correlation between listening scores and 
achievement scores in International Morse Code 
after 12 weeks of training was found to be 
—0.17. A correlation of 0.21 was found between 
listening scores and intelligence for a sample 
of 199 listeners, where the GCT scores were 
used as the measure of intelligence. When the 
listening scores were compared with scores on 
the test of memory span for digits, a correla¬ 
tion of 0.26 was obtained, and a similar study 
conducted by the Psychological Corporation 
found correlations of 0.16 and 0.20. 2 Observa¬ 
tions by the Psychological Corporation also 
gave correlations between talking and listening 
abilities of 0.27 and 0.41. In the light of this 
evidence, therefore, the conclusion seems well 
founded that listening in noise requires a spe¬ 
cial ability relatively unrelated to the other 
abilities discussed above and cannot be pre¬ 
dicted from other such tests. 


443 TRAINING 

A carefully devised program of practice and 
instruction very quickly raises the level of 
achievement of all operators, although the most 
efficient performance will be attained only by 
training those operators who have demon¬ 
strated the greatest native ability in communi¬ 
cation through preliminary selective tests. 

The rapid and extensive improvement with 
practice in the performance of a group of lis¬ 
teners can be ascribed primarily to the opera¬ 
tion of two factors. (1) The listeners learn to 
identify words partially masked by ambient 
noise or distorted by the characteristics of the 
particular equipment employed. (2) Experi¬ 
ence in the laboratory over a long period has 


shown that improvement can also be expected 
from the simplest instructions in the use of 
listening equipment. The listener should be 
shown how to fit the earphones most directly 
over the ear canal and how to adjust the head- 
band to provide the best acoustic seal against 
ambient noise. In addition, instructions in such 
matters as body tonus, fixation of attention, 
the major dialectical differences in pronunci¬ 
ation of standardized signal codes, and other 
devices have also been found to improve listen¬ 
ing performance. 

Speakers even more than listeners improve 
their performance with training. Instruction in 
the correct use of microphones will lead to a 
remarkable increase in the efficiency of com¬ 
munication. For example, novices in the use 
of hand-held microphones tend to hold the face 
of the microphone several inches from the lips 
and even experienced military fliers tell us that 
they habitually hold the microphone half an 
inch from the lips. Several experiments have 
demonstrated, however, that the nearer the car¬ 
bon type of microphone is held to the lips 
without interfering with the speech movements, 
the greater will be the intelligibility of the 
words spoken. Special instructions are also 
needed to insure the efficient use of throat and 
oxygen-mask microphones. For example, it has 
been found that novices tend to strap a throat 
microphone anywhere between the jaw and the 
base of the neck, although even a half-inch 
deviation from the correct position leads to 
marked deterioration in intelligibility. Also, 
some instruction in the most effective use of 
the voice for the particular purpose of convey¬ 
ing messages in noise will improve oral com¬ 
munication. The variables involved here are the 
same as those discussed in Section 14.1. 

One of the difficulties with training talkers 
to speak in noise is the difficulty in obtaining 
the proper equipment—amplifiers, noise gen¬ 
erators, etc.—with which to simulate the usual 
military situation. In order to meet this need, 
a portable interphone for rating and training 
talkers in noise was developed at the Psycho- 
Acoustic Laboratory and subsequently placed 
in production by the U. S. Navy (Special 
Devices Division). 0 The device permits two- 
way communication between talkers and in- 



TRAINING 


215 


structors either in quiet or in the presence of 
controlled amounts of interfering noise. The 
overall response of the interphone is essentially 
flat to 4,000 c, but filters are provided which, 
when switched in, introduce frequency distor¬ 
tion similar to that imposed by the resonant 
types of interphone and telephone equipment. 
A volume indicator makes it possible to meas¬ 
ure levels of both voice and noise. The inter¬ 
phone can drive either a loudspeaker or as 
many as 30 pairs of standard military ear- 



Figure 5. Photograph of training interphone 

phones. The complete instrument is contained 
in a convenient carrying case. Supplementary 
earphones, microphones, and cordage can be 
carried in a separate case. A photograph of 
this unit is given in Figure 5. 

The unit is adaptable both for rating talkers 
by the interview method and for instructing 
and training groups of talkers in voice proce¬ 
dures. The interphone makes it possible to 
standardize and introduce precision into rating 
and training routines conducted by adminis¬ 
trators stationed at different military centers. 
When used for classifying or rating military- 
telephone or radio-telephone talkers, the inter¬ 
phone can be used for two-way communication 
between the talker to be rated and any desired 
number of judges. The voice may then be eval¬ 
uated either in the presence or absence of noise 
or distortion. For purposes of training military- 
telephone talkers, for example, an entire class 


can listen over a loudspeaker or over earphones, 
while one or two talkers demonstrate proper 
procedures, correct use of the voice, and the 
effect of noise and resonance on intelligibility. 
Mockups of military communication networks 
can be set up and students can be drilled in 
quiet or in noise on the use of standard voice 
procedures. In addition, whenever precise meas¬ 
urements of the intelligibility of a talker are re¬ 
quired, the apparatus may be used to conduct 
standard articulation tests. 

The program established at the Submarine 
School at New London can be taken as an 
example of the type of training program which 
is possible. 9 In this particular instance, it was 
recommended that the training be given at 
three levels: (1) a basic course to teach the 
general skills in the use of telephone equipment, 
the ability to talk intelligibly over noise, and 
the observance of correct voice procedures and 
circuit discipline; (2) an intermediate course 
to give each man proficiency in the use of the 
particular phraseology required by his assign¬ 
ments aboard ship; and (3) an advanced course 
to drill the crew as a combat team in the co¬ 
ordinated use of the communication circuit. 

In the course of developing this program it 
was concluded that a short basic course (4 to 
6 hours) emphasizing correct enunciation, 
standard phraseology, and circuit discipline, 
produces a marked improvement in the ability 
of individuals to communicate intelligibly. 
More important, however, in increasing the 
overall efficiency of communications aboard 
ships are the intermediate and advanced 
courses. It is in these phases of the training 
that the application of standard procedures to 
specific jobs and the coordination of communi¬ 
cations for the crew as a whole are learned. 
Thus, the specific details of any given training 
course would necessarily be designed to meet the 
particular requirements of the services involved. 
In general, however, it may be said that the 
application of the principles discussed above 
within the framework of the particular situa¬ 
tion can be expected to produce rapid and ex¬ 
tensive improvements in communication per¬ 
formance. 









Chapter 15 

HEARING AIDS 


T he purpose of a hearing aid is to assist 
the hard-of-hearing patient to make the 
best use of his residual hearing. Thus a hearing 
aid should deliver to the patient’s ear speech 
sounds sufficiently amplified and of appropriate 
quality to be intelligible in spite of his hearing 
loss. It should operate effectively over a wide 
range of input levels, without exposing the 
patient to possible discomfort from excessively 
loud sounds or to unnecessary annoyance from 
extraneous noise. 

The discussion of hearing aids can be divided 
into four related problems: (1) the measure¬ 
ment of hearing loss, (2) the evaluation of 
hearing-aid performance, (3) the selection of 
a hearing aid for the patient, and (4) the de¬ 
sign recommendations for future hearing aids. 


151 METHODS OF MEASURING 
HEARING LOSS 

Any “rating” of auditory function by a single 
number will always necessitate some simpli¬ 
fying assumption, either in the construction of 
a simple test or in the interpretation of an 
elaborate test. At least two very different re¬ 
quirements are involved in the objectives of 
different auditory tests. One is the selection of 
listeners where the objective is to be sure that 
all who pass the test have at least “normal” 
auditory function over a wide band of frequen¬ 
cies. The other is the detection and evaluation 
of impairment of hearing for speech to deter¬ 
mine the necessity for special attention, for 
discharge from service, or for aural rehabili¬ 
tation. It would be fortunate, indeed, if a single 
test sufficed for both purposes. Theoretically, 
the sensory functions involved might perfectly 
well differ as much from one another as visual 
acuity differs from color blindness. 8 

Because of its value to the diagnostician, the 
pure-tone audiometric test has for many years 
been the accepted test of hearing loss. In many 
cases, however, it is valuable to have a direct 
estimate of the hearing loss for speech. Two 


tests, identical in design and in method of ad¬ 
ministration, have been developed by the 
Psycho-Acoustic Laboratory for this purpose. 
The tests differ only in the nature of individual 
items. They are phonographically recorded, and 
may be administered to a single subject or to a 
group of subjects having similar audiograms. 

An important characteristic of the two tests 
is that the items are homogeneous with regard 
to difficulty for normal ears. Homogeneity is 
important for two reasons. (1) The structure 
of the test is such that the items are divided 
into subgroups and each subgroup recorded at 
progressively lower levels of intensity. Thus, 
in rescrambled versions of the tests, the basic 
difficulty is not changed. (2) When the items 
are essentially homogeneous, all the items 
are understood within a relatively narrow range 
of intensities above the threshold of detecti- 
bility. This is a desirable feature, since it in¬ 
sures that the function relating the percentage 
of the items correctly heard to the intensity 
at which they are presented will be steep, and 
an arbitrary threshold for the test can be ex¬ 
pressed in decibels with a minimum of vari¬ 
ability. 

Auditory Test No. 9 4 consists of two lists of 
42 dissyllabic words of the spondaic stress pat¬ 
tern, i.e., words such as “railroad” and “grey¬ 
hound,” in which both syllables are accented. 
The words are divided into seven groups of six 
words each, and each group is recorded at 
progressively lower intensity levels 4 db apart. 
Auditory Test No. 12 5 consists of eight lists of 
short, simple questions. Each list is composed 
of 28 items which are divided into seven groups 
of four items. Each group of four items is 
recorded at an intensity level 4 db lower than 
the preceding one. In the selection of items for 
Test No. 12 an effort was made to have the 
sentences short, simple, representative of Eng¬ 
lish sounds, typical of conversational speech, 
and homogeneous with respect to recognizabil- 
ity for normal listeners. Homogeneity for the 
sentences was achieved by determining the 
threshold of intelligibility for all the sentences 


216 


METHODS OF MEASURING HEARING LOSS 


217 


and then recording each sentence at a level so 
adjusted as to equate all the sentences in the 
group for average intelligibility. 

The hearing loss for any particular subject 
is taken as the difference between his threshold 
of hearing on either of the two tests and the 
average threshold established under the same 
conditions for a group of normal subjects. As 
long as the same playback system and testing 
situation are maintained, it is possible to de¬ 
termine a relative intensity value for the thresh¬ 
old of hearing for speech. This objective may 
be achieved satisfactorily even though nothing 
is known about the “absolute” speech intensities 
involved. 

The obvious shortcoming of such a speech 
test of hearing loss is that, taken by itself, no 
indication is given as to whether the hearing 
loss is due to inability to hear high frequencies 
or low frequencies. An experimental attempt 
was made, therefore, to show a diagnostic dif¬ 
ference between uniform and high-frequency 
losses for speech by the use of filters in the 
speech circuit. The words used in Auditory Test 
No. 9 were passed through a high-pass circuit 
which cut off at 4,000 c and filtered the fre¬ 
quencies below 4,000 c at the rate of 17 db per 
octave. Since the frequencies below 4,000 c are 
considerably attenuated, the listener is forced 
to use the higher frequencies of the speech in 
order to interpret the sounds. This test was 
given to both normal and high-tone hard-of- 
hearing subjects. The preliminary experiments 
showed that persons whose audiograms are 
flat, regardless of the degree of loss, suffer the 
same loss on the filtered as on the unfiltered 
version of the test. However, patients who have 
hearing losses primarily for the high frequen¬ 
cies hear fewer words with the filtered than 
with the unfiltered form of the test. One of the 
chief troubles with this test in its present form 
is that, although the words used are homogene¬ 
ous in their undistorted form, their homogene¬ 
ity decreases sharply when they are put through 
a high-pass filter. Consequently, a much wider 
range of intensities is needed to cover the range 
from zero to maximum intelligibility, and 
greater variability is obtained. 


15 2 EVALUATION OF HEARING-AID 
PERFORMANCE 

A major part of the investigation of hearing 
loss was concerned with the experimental study 
of hearing aids, both as physical instruments 
and as aids to hearing. The physical aspects 
were studied at the Electro-Acoustic Labora¬ 
tory and the psychological aspects at the 
Psycho-Acoustic Laboratory. 

15.2.x Electro-Acoustic Measurements 

Ideally, the performance of a hearing aid as 
measured by physical laboratory tests should 
represent the performance of the aid when worn 
by an average person under normal conditions. 
At the same time, it is highly desirable that 
laboratory test methods and equipment be as 
simple and easily duplicable as possible. In prin¬ 
ciple, it is feasible to make meaningful physical 
measurements of the performance of hearing 
aids worn by human talkers and listeners. The 
concepts outlined in Chapter 9 for interphone 
systems could be applied and performance could 
be measured relative to the orthotelephonic 
reference system. Physical measurements of this 
type are, as a rule, neither simple nor easily 
performed, and since the conditions of use of 
a hearing aid vary widely, a comprehensive 
testing program of this type would be imprac¬ 
tical. 

The physical test methods for hearing aids 
now in common use are the result of a compro¬ 
mise. They involve, in general, the mounting 
of the hearing aid in a known sound field pro¬ 
duced by a loudspeaker (artificial voice) in a 
room free from acoustic reflections, and the 
measurement of the characteristics of the sound 
pressure developed in a closed cavity (artificial 
ear) by the hearing-aid earphone. 

These methods, along with the results of 
measurements on a large number of hearing 
aids, are described in an extensive report. 7 The 
methods used represent a combination of pro¬ 
cedures developed at the Electro-Acoustic Lab¬ 
oratory with those discussed in a report 3 and 
those established by the Committee of the 
American Hearing Aid Association. 6 They have 
the advantage of measuring the performance 



218 


HEARING AIDS 


of the aid under simplified conditions which can 
be easily standardized. On the other hand, con¬ 
ditions of use are approximated only in a gen¬ 
eral way. Consequently, additional experiments 
must be performed to determine the corrections 
to be applied to the laboratory results to take 
into account the effect of conditions of use and 
to cover the range of variability which may be 
expected. A number of such experiments are 
discussed at the end of this section. 


ceiver coupled to a carbon transmitter) may be 
inserted between the microphone and earphone 
to provide increased amplification. The vacuum- 
tube aid consists of a microphone, usually of 
the crystal type, an electronic amplifier, and an 
earphone, usually of the crystal or magnetic 
type. Portable batteries supply the operating 
power. 

Against the somewhat decreased weight, 
lower battery requirements, and greater sim- 




Figure 1. Clamp for electro-acoustic tests on hearing aids. 


Present-day, wearable hearing aids of the 
air-conduction type a are generally of either the 
carbon or the vacuum-tube variety. The carbon 
aid is identical in principle with the ordinary 
telephone. It consists of a carbon microphone 
(transmitter) and a magnetic earphone (re¬ 
ceiver), with operating power supplied by 
portable batteries. A carbon amplifier (a re- 

a Bone-conduction aids do not form part of this dis¬ 
cussion. 


plicity of the carbon aid, the vacuum-tube hear¬ 
ing aid has the following advantages. 

1. Larger gain. 

2. Increased frequency range. 

3. Better and more flexible control of the fre¬ 
quency-response characteristic (tone control). 

4. Better stability. 

5. Absence of noise inherent in carbon-type 
microphones. 

It is for these reasons that the vacuum-tube 



















EVALUATION OF HEARING-AID PERFORMANCE 


219 


air-conduction hearing aid is now in most ex¬ 
tensive use. 

In the laboratory, the hearing aid under test 
was mounted in a small scissors-type clamp 
which was elastically suspended by rubber 
bands from a wire bent into the shape of a U 
(see Figure 1). Clamp and aid were located 
facing a loudspeaker at a distance of 1 m in a 
room free from acoustic wall reflections [ane- 
choic chamber (see Chapter 10)]. The loud¬ 
speaker (artificial voice) was either a Western 
Electric Type 751-B (see Figure 2) or a West- 



Figure 2. Artificial voice (WE Type 751-B). 

ern Electric Type 750-A unit (see Figure 3) 
mounted in a long tube lined with absorbent 
material. The response characteristic of one of 
these units is given in Figure 4. Driving voltage 
for the loudspeaker was obtained from a power 
amplifier, driven in turn by an oscillator whose 
frequency could be varied over the audible fre¬ 
quency range. 

The earphone was located in a control room 
adjacent to the anechoic chamber and was 
coupled to a rigid cavity of about 2 cu cm 
volume which acoustically simulated a human 
ear into which a conventionally molded Lucite 
earpiece had been inserted. 3 * 1 The sound pres¬ 
sure developed by the earphone in the cavity 
was measured by a sound-pressure meter (see 
Chapter 10) using a Western Electric Type 
640-A or 640-AA condenser microphone. A 


cross section of the coupler used by the Electro- 
Acoustic Laboratory is shown in Figure 5. 

The same sound-pressure meter and micro¬ 
phone were used to set up a known free-field 
sound pressure, p lt in the anechoic chamber 

mm r : ’ 'M 



Figure 3. Artificial voice with acoustic ter¬ 
mination (WE Type 750-A). 

at the location of the hearing aid. If the hearing 
aid is then introduced into the sound field and 
the sound pressure p., is measured in the arti- 



IOO 1000 10,000 


FREQUENCY IN CYCLES PER SECOND 

Figure 4. Frequency-response characteristic of 
artificial voice. 

ficial ear, the overall acoustic gain of the aid is 
defined as 

Acoustic gain in db = 20 log (1) 

This procedure is schematically illustrated in 
Figure 6. With this general setup, more than 
thirty commercial hearing aids underwent the 
battery of tests briefly described below. 

Frequency-Response Characteristic 
With a free-field input sound-pressure level 






































220 


HEARING AIDS 


of 60 db b the gain control of the aid was ad¬ 
justed so that a sound-pressure level [SPL] 
of about 100 db was reached at the frequency 
of maximum acoustic gain. In order to simplify 
comparison of the shapes of response charac¬ 
teristics of different hearing aids, the curves 
were plotted relative to the response at 1,000 c. 
This was done for various representative set¬ 
tings of the tone control. 

Output versus Input Characteristic 

To determine the range of linear operation 
of the aid the output sound-pressure level was 
determined for input sound-pressure levels of 



Figure 5. Artificial ear (volume 2 cu cm). 


40, 50, 60, 70, 80, and 90 db and maximum gain- 
control setting. This was done for the following 
frequencies: 600, 1,000, 2,000, 3,000 c, and the 
frequency of maximum acoustic gain. The out¬ 
put sound pressures are plotted against the 
input sound pressures, with frequency as the 
parameter. 

Nonlinear Distortion Characteristics 

The gain control was set corresponding to an 
acoustic gain of 20 db and a free-field input 
sound-pressure level of 60 db was established. 
The output voltage of the artificial ear was 
analyzed by a narrow-band harmonic analyzer. 
From measurements of the fundamental and 
the harmonics, a single figure was computed in 
the conventional manner as the measure of 
harmonic distortion. Various volume-control 

b All sound-pressure levels are expressed in decibels 
with reference to 0.0002 dyne/cm 2 which is the refer¬ 
ence level standardized by the American Standards 
Association. 


settings were used with a range of input levels 
up to 90 db, at frequencies of 600, 800, and 
1,000 c. 

Miscellaneous Tests 

The following tests were also performed: 
variation of gain with battery voltage, meas- 



DESIRED FREE-FIELD SOUND PRESSURE AT POINT A IS ESTABLISHED 
WITH THE AIO OF THE SOUND PRESSURE LEVEL METER. ( p,) 



KNOWN FREE-FIELD SOUND PRESSURE LEVEL p, AT POINT A. 

SOUND PRESSURE LEVEL p 2 DEVELOPED BY EARPHONE IN COUPLER. 

20 LOG P 2 /P, = OVERALL ACOUSTIC GAIN OF HEARING AID IN DB 

Figure 6. Measuring the overall acoustic gain of 
a hearing aid. 

urement of battery drain, and effect of temper¬ 
ature and humidity. 

Tests of Component Parts 

It is frequently desirable to know the acoustic 
and electric characteristics of the microphone, 
amplifier, and earphones as individual units. 
The techniques of measurement are very sim¬ 
ilar to those described above for the overall 
characteristics of the complete hearing aid. In 
the case of the microphone, the voltages devel¬ 
oped by the unit for various known free-field 
sound-pressure inputs are measured. Measure- 































































EVALUATION OF HEARING-AID PERFORMANCE 


221 


ments of amplifier characteristics are purely 
electrical and present no special problems. For 
earphones, the sound pressures developed in 
the artificial ear are determined for specified 
electric inputs to the earphone. Two useful 
ways of specifying the electric signal to an ear¬ 
phone are either in terms of constant voltage 
or in terms of 1 mw constant power available 
(see Chapter 10). The absolute value of the 
electric impedance of the earphone, terminated 
acoustically by the artificial ear, is also a useful 
quantity. 

Conclusions 

From the extensive physical tests of com¬ 
mercial hearing aids conducted at the Electro- 
Acoustic Laboratory, the following general con¬ 
clusions can be drawn. 

1. The frequency-response characteristics are 
peaked. The most prominent peak is usually 
caused by the earphone characteristic and oc¬ 
curs in the frequency region from 1,000 to 
2,500 c. Secondary peaks may arise from both 
earphone and microphone characteristics. 

2. Tone controls are often ineffective, acting 
only as an additional gain control without 
markedly affecting the shape of the response 
characteristic. 

3. Nonlinear distortion generally decreases 
with increasing frequency of the signal for 
given conditions of input signal and volume 
control. 

In Figures 7 to 9 characteristics of three 
representative hearing aids are given; two are 
of the vacuum-tube type and one of the carbon 
type. 

Performance Characteristics of Hearing 
Aids under Conditions of Use 

The physical tests outlined above can be used 
to good advantage for comparison of different 
makes of hearing aids. They yield only a rough 
approximation, however, to the characteristics 
obtaining under “normal” conditions of use. 
A series of supplementary measurements are 
described below which are useful in determin¬ 
ing the nature and range of the effects caused 
by the difference between laboratory and use 
conditions. The principal differences are due to 
the following facts. 


1. The artificial ear does not duplicate acous¬ 
tically the human ear with earpiece. 

2. The fit of individually molded earpieces to 
the wearer’s ear and to the earphone varies. 

3. The body of the wearer acts as an acoustic 
“baffle.” 

4. Sometimes the microphone of the aid is 
covered by clothes. 

5. The acoustic environment and the sound 
fields to which the wearer is exposed are dif¬ 
ferent from the sound field in the anechoic 
chamber. 

A direct comparison of the response charac¬ 
teristics of various earphones on the coupler 
and on the ear may be made if means are 
provided for measuring the sound pressure 
which the earphones develop in the ear canal 
at the tip of the earpiece with which the ear¬ 
phone is worn. This is done by attaching a 
probe tube to the Type 640-AA condenser mi¬ 
crophone of the sound-pressure meter with 
suitable coupling rings. The probe tube was 
molded into the earpiece with its opening just 
at the edge of the channel transmitting the 
sound from the earphone (see Figure 10). The 
probe tubes used were about 3 to 4 in. in length, 
with a bore of 0.025 in. Considerable attenua¬ 
tion occurs when sound is transmitted through 
the tubes, and each tube must be calibrated 
individually. A typical correction curve to be 
applied to the calibration of a condenser micro¬ 
phone when used with a probe is shown in 
Figure 11. 

To obtain reproducible conditions of good 
seal, the earphone was sealed to the earpiece 
which in turn was sealed into the subject’s ear 
with a mixture of beeswax and lanolin. The 
same earphone was also tested on the coupler. 
The ratio of the sound pressure developed in 
the ear to the sound pressure developed in the 
coupler is plotted, in decibels, in Figure 12 for 
three earphones. The results have been averaged 
for three subjects and have been labeled 
“coupler correction” for brevity. It is clear that 
this correction is a strong function of the type 
of earphone. 

Figure 13 shows the average ratio (leakage 
correction) of the sound pressure as measured 
by the probe tube with and without the sealing 
agent (lanolin and wax). The results of several 



222 


HEARING AIDS 


earphones were averaged and plotted for three given sound field will also be affected by the 
observers. If earpieces are used which are not presence of sound-reflecting surfaces nearby, 
individually fitted to the wearer, considerably In the standard laboratory tests, described at 



FREQUENCY IN CYCLES PER SECOND 




FREQUENCY IN CYCLES PER SECONO 






Figure 7. Performance characteristics of hearing aid X. 


greater values of leakage correction are to be 
expected. 

The response of a hearing aid placed in a 


the beginning of this section, care was taken 
that there be no such disturbing surfaces. In 
actual use the hearing aid is worn on the per- 



















































































































































































































































EVALUATION OF HEARING-AID PERFORMANCE 


223 


son, who presents a sizable reflecting surface body-baffle correction by measuring the re- 
or acoustic “baffle” directly in back of the aid. sponse of a given aid in the free sound field 
The position on the person in which the aid is and with a number of subjects wearing it. The 




RESPONSE WITH 1 MILLIWATT CONSTANT POWER AVAILABLE 






40 


50 


60 


70 


*0 


U) 

o 


<r 

UJ 

CL 



0 --- 1 - 

60 80 100 120 140 


OUTPUT SOUND PRESSURE IN OB 



40 SO 60 70 80 90 


INPUT FREE-FIELD SOUNO PRESSURE LEVEL IN OB INPUT FREE-FIELD SOUNO PRESSURE LEVEL IN 09 



Figure 8. Performance characteristics of hearing aid Y. 


worn, the size and shape of the individual, the 
nature of the clothing worn, etc., will affect 
the response characteristic. 

Measurements were made to determine this 


acoustic effect of the human body can be broken 
down into three parts. First, there is an in¬ 
crease in response of about 5 db at low frequen¬ 
cies, falling off to zero between about 600 and 
































































































































































































































































































224 


HEARING AIDS 


1,000 c. Second, there are one or two marked 
dips in the frequency range between 1,000 and 
2,500 c. This dip is a strong function of the 



FREQUENCY IN CYCLES PER SECOND 



OUTPU 

CHARA 

VS INPUT 
CTERISTIC 

300 

600 

1000 

000 







/ 














/ 



/ 

/ 

/ / 
/ / 



/ 

/ / 

/ 

/ / 




/ 

/ 

/ 

/ / 




40 50 60 70 80 90 


100 1000 2000 INPUT FREE-FIELD SOUND 

FREQUENCY IN CYCLES PER SECOND PRESSURE LEVEL IN DECIBELS 


Figure 9. Performance characteristics of hear¬ 
ing aid Z. 


spacing between body and aid. Third, the re¬ 
sponse above about 2,500 c is not affected sig¬ 
nificantly. Figure 14 shows typical results for a 
given aid and four subjects. 



Figure 10. Molded earpieces with probe tubes. 

Figure 15 shows typical corrections for the 
case where a suit coat is worn over the aid. 

To investigate the magnitude and character 


of the acoustic body-baffle effect of the wearer’s 
person on the response of a hearing aid in more 
detail, measurements of this effect were also 
carried out in a “random” sound field. The sub¬ 
jects were located in a room with hard walls 



FREQUENCY IN CYCLES PER SECOND 

Figure 11. Typical microphone correction for 
probe tube. 

equipped with polycylindrical protrusions to 
insure that sound waves of about equal intensi¬ 
ties impinged on the wearer from all directions. 

The body-baffle effect so determined was of 
considerably smaller magnitude than the one 
determined in the anechoic chamber. These two 
conditions of acoustic environment encompass 



FREQUENCY IN CYCLES PER SECOND 

Figure 12. Coupler correction. 


about the range encountered in use. Tests con¬ 
ducted at the Psycho-Acoustic Laboratory in¬ 
dicate that the body-baffle effect is not impor¬ 
tant, insofar as the intelligibility of speech is 
concerned. Hence it can be concluded that the 
body-baffle effect measured under free-field con¬ 
ditions is not representative of most conditions 
of use. 
































































































































































































EVALUATION OF HEARING-AID PERFORMANCE 


225 


The effects of environment are perhaps the 
largest correction to be applied to the response 
characteristics obtained by the physical meth¬ 
ods outlined at the beginning of this section. 



Figure 13. Leakage correction. 

These measurements, as will be recalled, were 
carried out in the very specialized acoustic en¬ 
vironment of the anechoic chamber. This was 
done in order to investigate the electro-acoustic 
performance of the instruments in a known 



FREQUENCY IN CYCLES PER SECOND 

Figure 14. Body-baffle correction. 

and easily specifiable sound field. Under actual 
conditions of use, however, the instrument will 
usually be worn in rooms which are not highly 
soundproofed and, furthermore, the wearer of 
the aid may very frequently not be facing the 
source of sound to which he is listening. 

It should be emphasized that in normal use 
the character of the received sound will be 


strongly dependent on the acoustical character¬ 
istics of the surroundings. At present no data 
are at hand which indicate what the differences 
may be between the response characteristics 
measured by the standard laboratory procedure 
and the apparent characteristics when the hear¬ 
ing aid is worn in an “average” room or office. 

The difference in performance of the hearing 
aid under laboratory conditions and under con¬ 
ditions of actual use are sufficient in magnitude 
to warrant serious consideration. Because of 
the wide range of such variations, electro¬ 
acoustic performance data taken by the stand¬ 
ard laboratory procedures should be applied 
with caution to the problem of “fitting” a hear¬ 
ing aid to an individual. The character and 
magnitude of the possible differences in the 
electro-acoustic performance of an instrument 
when it is worn by different persons under 
various conditions should be borne in mind in 
the design of the aid. Sufficient gain, power- 
output capacity, and control of frequency- 
response characteristics should be provided, if 
possible, to allow the wearer to adjust the aid 
for good operation for the great variety of con¬ 
ditions encountered. 



100 1000 10,000 
FREQUENCY IN CYCLES PER SECOND 

Figure 15. Clothing correction. 

15 2 2 Psycho-Acoustic Measurements 

In evaluating the intelligibility of speech 
heard via a hearing aid, it would be desirable 
to use a crew of listeners with impaired hear¬ 
ing, all of whom had the same type and same 
degree of hearing loss. The difficulty of obtain¬ 
ing such a crew is readily apparent in view of 

























































































































































226 


HEARING AIDS 


the wide variation in the severity of naturally 
occurring or traumatic deafness. For this rea¬ 
son, it was decided at the inception of the 
psycho-acoustic study to employ a crew of 
trained listeners with normal hearing and to 
render them functionally hard-of-hearing dur¬ 
ing the tests by means of masking noise. The 
theoretical and experimental bases for this pro¬ 
cedure are discussed in reference 1. A flat 
spectrum of random noise was used to produce 
a simulated hearing loss of equal severity for 
all subjects. The amount of loss was easily 
varied by increasing or decreasing the intensity 
of the noise. The degree of hearing loss for 
pure tones occasioned by the various levels of 
masking noise was measured, but a more gen¬ 
eral and equally valid expression for the amount 
of hearing loss was obtained by determining 
the effective hearing loss in decibels for speech. 

The instruments selected for the tests were 
all of the vacuum-tube type; they were products 
having the approval, as of January 1944, of 
the Council of Physical Medicine of the Ameri¬ 
can Medical Association. The instruments were 
purchased from the stock of local dealers. 
Where several models of the same make were 
available, only those models were chosen for 
test which, according to the dealers’ own state¬ 
ments, were most frequently recommended for 
hard-of-hearing patients. 

Each individual model was tested with two 
or more earphone types when available and 
with minimum and maximum volume-control 
settings. In the case of instruments with varia¬ 
ble tone control the setting giving the most 
nearly flat response characteristic and that 
giving greatest emphasis to the high tones were 
tested. It was not feasible to study all the pos¬ 
sible combinations of the three factors. 

The hearing aid to be tested was elastically 
suspended on a ring stand in an anechoic box, 
and connected speech was presented to it from 
a loudspeaker 20 in. distant. For most of the 
tests the speech used was phonographically re¬ 
corded. Analysis of the spectrum of this speech 
signal showed it to be identical with the spec¬ 
trum obtained when the live voice was substi¬ 
tuted for the phonograph and loudspeaker. The 
earphone of the hearing aid was coupled to a 
condenser microphone, the output of which was 


distributed, essentially unchanged, through 
standard-sized earphones of high fidelity, to 
four subjects who were seated in a quiet, sound- 
treated room. The masking noise was intro¬ 
duced electrically into the subjects’ earphones 
through a separate channel. A block diagram 
of the testing equipment is shown in Figure 16. 



DISTRIBUTING CIRCUIT (ACOUSTIC GAIN* 0 OB) 


Figure 16. Block diagram of the circuits used 

for testing hearing aids. 

One aspect of the performance of the hearing 
aids was evaluated in terms of the speech-input 
level at the hearing-aid microphone required 
to give sufficient output to override various 
degrees of simulated hearing loss. By plotting 
the speech-input level required to reach the 
threshold of intelligibility at each level of the 
effective hearing loss, a function bounding 
the area of intelligibility for speech for a given 
instrument was obtained. 73 From such a func¬ 
tion the minimum speech level which can be 
made intelligible for a given degree of hearing 
loss may be read. This speech level is deter¬ 
mined primarily by the effective gain of the 
instrument. The maximum hearing loss which 
can be overcome at any voice level may also be 
read from the curve. This value depends pri¬ 
marily on the maximum power output of the 
hearing aid. 

Sample plots of this nature are shown in 
Figures 17 and 18. The area of intelligibility 
for speech is bounded above and on the left 
by the curve representing the threshold of in- 


























































EVALUATION OF HEARING-AID PERFORMANCE 


227 


telligibility. The position of this threshold 
curve varies with the volume-control setting. 
The upper limit, or “ceiling,” is imposed pri¬ 
marily by the maximum power output of the 
instrument. As a reference, a high-fidelity sys¬ 
tem was substituted for the hearing aid, and 
the amplifiers were so adjusted that no acoustic 
gain was introduced. The area of intelligibility 
is bounded below and on the right by the thresh¬ 
old for this “no-gain” condition. The effective 
gain of the hearing aid is then measured, as 
shown in Figures 17 and 18, as the number of 
decibels between the threshold-of-intelligibility 
curve for the reference system and the thresh¬ 
old-of-intelligibility curve obtained when the 
hearing aid is inserted. 

The quality of speech reproduced by the 



Figure 17. Showing threshold of intelligibility 
at different speech levels with the volume-control 
setting as parameter. Data for hearing aid X. 


hearing aid was also studied. The listeners 
were asked to compare the speech transmitted 
by each instrument with (1) speech of high 
fidelity and (2) speech which was greatly dis¬ 
torted but still intelligible. A rating on a seven- 
point scale of quality was assigned to each 
instrument. These tests were made at a com¬ 
fortable output level without the use of simu¬ 
lated hearing loss. There is a significant positive 
correlation (0.62) between the quality ratings 
and the number of decibels above the limit of 
linear amplification at which the rating was 
made. It appears, therefore, that the quality 
rating depended largely on the degree and char¬ 
acter of the limiting and consequent harmonic 
distortion that was present. 

There are countless minor differences be¬ 


tween instruments, but all those examined were 
enough alike to make it doubtful whether any 
of the untested instruments would provide great 
advantages based on radical differences in de¬ 
sign. For low and medium input levels, output 
is proportional to input (linear amplification), 
and the amount of gain is regulated by the gain 
or volume control. The maximum acoustic out¬ 
put of the hearing aid is definitely limited, 
usually by the power capacity in the final stage 
of the amplifier. All instruments introduced a 
considerable degree of frequency distortion. 
Below about 500 c and above about 2,500 c, the 
amplification of most instruments falls sharply. 
All the powerful instruments were subject to 
squeal from acoustic feedback if they were used 
at high gain with a poorly fitting earpiece. 



Figure 18. Showing threshold of intelligibility 
at different speech levels with the volume-control 
setting as parameter. Data for hearing aid Y. 


None of the hearing aids, when set for maxi¬ 
mum volume, caused painful stimulation of the 
normal listener’s ears, even when the speech- 
input level was equivalent to 100 db sound- 
pressure level. Wide differences between instru¬ 
ments are found in terms of the lowest input 
levels at which speech can be made intelligible. 
With an effective hearing loss of 75 db, the 
range of minimum speech levels is from 29 to 
80 db for different instruments. The hearing 
aids vary much less in terms of the maximum 
effective hearing loss which they are able to 
overcome at high speech levels. With sufficient 
speech-input level the weakest instrument is 
able to give intelligible speech for a hearing 
loss of 75 db, the most powerful for a hearing 
loss of 90 db. 
















228 


HEARING AIDS 


15 3 SELECTION OF A HEARING AID 

The selection of a hearing aid for any partic¬ 
ular patient is a difficult problem because of 
the variety of requirements for a hearing aid, 
the variety of existing instruments, and the 
variety of individual auditory impairments. It 
is impossible to base the selection of hearing 
aids on any single or composite “figure of merit” 
derived from purely physical measurements. 
No single physical characteristic is sufficiently 
all-important to be made the basis of any such 
figure of merit, and it is unlikely that any 
workable formula for combining the values of 
several characteristics will be found. 

The primary requirement is, of course, that 
the aid make speech intelligible to the patient. 
In most cases, however, several different aids 
will satisfy this requirement equally well, and 
the problem is to choose among them. At this 
point several secondary criteria become impor¬ 
tant to the patient: comfort, weight, expense, 
reliability, serviceability, quality of the sound, 
etc. On these matters the patient’s personal 
opinion is the chief criterion. 

For many years the principle of “selective 
amplification” has been widely accepted, im¬ 
plicitly or explicitly, as a guide to the selection 
of the best hearing aid. The classical argument 
runs somewhat as follows. A patient’s hearing 
loss is not necessarily uniform as a function 
of frequency. Many patients become deaf to 
high-frequency sounds but still hear sounds of 
low frequency quite well. A smaller number suf¬ 
fer from low-tone deafness, while for a few the 
tones in the middle of the audible range are 
most impaired. A very common type of hearing 
loss is moderate for intermediate frequencies 
and severe or total for sounds of high pitch. 
Now the various alternative aids are also 
known to differ, particularly in respect to their 
frequency-response characteristics. Some em¬ 
phasize high tones more than others, some 
amplify a wider band of frequencies (see Sec¬ 
tion 15.2.1). It is “obvious,” therefore, that the 
instrument which restores the patient’s audio- 
gram most nearly to normal is the best for that 
patient. 

The principle of restoring the audiogram to 
normal by selective amplification is used as a 


guide in selecting the particular earphone or 
tone-control setting, and is made the basis of 
a specific test, the so-called “aided audiogram,” 
to determine which instrument best fits the 
patient. The method is often compared to the 
fitting of eyeglasses to correct defects of vision. 

The various objections to the procedure of 
“audiogram fitting” may be summarized. 10 The 
audiogram reveals the threshold sensitivity, but 
listening is usually done well above the thresh¬ 
old level. It has been suggested, 2 accordingly, 
that it is an equal-loudness contour at the level 
of comfortable listening that should be fitted 
rather than the threshold. Unfortunately, it is 
impractical to determine equal-loudness con¬ 
tours as a routine procedure with untrained 
patients. The attempt was made at the Deshon 
General Hospital as a part of the present 
project, and the determination proved to be 
either unreliable or unduly time-consuming. 

The relative intelligibility obtained with sev¬ 
eral widely different patterns of frequency 
response was determined at the Psycho-Acoustic 
Laboratory for 25 hard-of-hearing ears. Of 
these, four could be classified as having low- 
frequency losses, five as having uniform loss 
at all frequencies, ten showed a gentle slope 
downward at high frequencies, and six had 
marked high-frequency losses. This classifica¬ 
tion of the shapes of the audiograms is admit¬ 
tedly arbitrary, but corresponds roughly to 
“audiogram-fitting” practices. The frequency 
responses of the testing equipment included, 
in addition to the flat system, four “tilts,” both 
rising and falling with frequency at rates of 
6 or 12 db per octave (see Figure 19 in Chapter 
7). These frequency responses will be referred 
to as LP-12 (low-pass, sloping at 12 db per 
octave), LP-6, flat, HP-6 (high-pass, rising at 
6 db per octave), and HP-12. Articulation tests 
were run with each patient for various condi¬ 
tions of frequency distortion, and the results 
permit the conditions to be rank-ordered for 
each patient. Two criteria were used as the 
basis for evaluating the subject’s performance 
on the several frequency patterns. 

Maximum Articulation Score 

The rank orders obtained on the basis of the 



DESIGN OBJECTIVES FOR HEARING AIDS 


229 


maximum articulation score for each frequency 
pattern may be summarized as follows: 

LP-12. Only two subjects were able to obtain 
articulation scores higher than 50 per cent and 
both cases were much more successful with the 
other conditions. The inferiority of this fre- 
qency pattern is clearly indicated. 

HP-12. Two of 14 ears did as well with this 
frequency pattern as with any other, but for 
at least 10 subjects this pattern was rated as 
third, fourth, or fifth best. 

LP-6. Three out of 22 did as well on this pat¬ 
tern as on any other, but most of the scores 
(17 cases) ranked the pattern as the third or 
fourth choice. In no case did the LP-6 give a 
better score than the flat or the HP-6. 

HP-6. Data were available with this fre¬ 
quency pattern for all 25 ears. Twenty-three of 
these obtained scores which were ranked first, 
and the remaining two ranked this frequency 
pattern as second best. However, 18 of the 23 
firsts were shared equally with the flat system. 

Flat. For 21 of the 25, the score on the flat 
frequency pattern was assigned the rank of 1. 
The flat ranked second to the HP-6 in four 
cases, third in one case. 

15.3.2 Maximum Operating Range 

The operating range is defined as the range, 
in decibels, between the lowest speech-input 
level at which the listener attains an articula¬ 
tion score of 50 per cent and the highest level 
at which nonlinear distortion again reduces the 
score to 50 per cent. Nonlinear distortion (peak 
clipping) was used to protect the listener, and 
in all cases the distortion began at the same 
peak intensity (124-db SPL) in the listener’s 
ear. 

The maximum articulation scores and the 
operating range are not independent since they 
are both derived from the same articulation 
functions. In general, the two variables are 
highly correlated, but they may disagree under 
some circumstances. The degree to which the 
ranks assigned on the basis of the operating 
ranges agree with ranks similarly assigned on 
the basis of the maximum scores can be sum¬ 
marized as follows: 


Flat: rankings agree in 19 of 24 cases. 

HP-6: rankings agree in 24 of 24 cases. 

LP-6: rankings agree in 14 of 21 cases. 

HP-12: rankings agree in 8 of 13 cases. 

LP-12: rankings agree in 2 of 2 cases. 

Thus the agreement is relatively close be¬ 
tween these two measures of performance, and 
the conclusions drawn from maximum articu¬ 
lation scores are supported by the data on the 
operating ranges. It is clear that the perform¬ 
ances of the HP-6 and flat frequency patterns 
head the list. In every one of the 25 cases either 
the flat or the HP-6 ranked first. None of the 
other three patterns competes seriously with 
these two. 


1S-3 ' 3 Quality Preferences 

For 23 of the 25 ears studied, the listeners 
made subjective ratings of the quality of the 
speech. In 14 cases the preferred quality was 
produced by the same frequency pattern which 
gave the best performance on the articulation 
tests, but in 9 instances the quality judgment, 
was either ambiguous or definitely misleading- 
In some cases, therefore, the patient’s personal' 
opinion about a secondary requirement should 
give way to the primary requirement of intelli¬ 
gible speech. 

These results, definitely contrary to the orig¬ 
inal expectations of the experimenters, seem to 
show that it is possible to specify the desirable 
frequency characteristic of a hearing aid more 
successfully by a simple, general rule than by 
any interpretation of the patient’s audiogram. 


154 DESIGN OBJECTIVES FOR HEARING 
AIDS 

On the basis of the data obtained on hard-of- 
hearing subjects, tentative design objectives 
can be drawn up. 10 Design problems are greatly 
simplified by the realization that a single, versa¬ 
tile instrument of appropriate general charac¬ 
teristics, with a variable tone control and a 
semipermanent adjustment of maximum acous¬ 
tic output can encompass the entire range of 
requirements. 

The overall acoustic frequency characteristics 



230 


HEARING AIDS 


of a hearing aid should be uniform (without 
sharp peaks) between 400 c (preferably 300 c) 
and 3,000 c (preferably 4,000 e). The response 
contour should be smooth at the maximum 
power-output level as well as at ordinary oper¬ 
ating levels. The high-frequency cutoff can be 
as abrupt as engineering convenience requires. 
Below 300 c the frequency response should fall 
off at a rate of at least 10 db per octave and a 
sharper cutoff is permissible. 

Between the cutoff frequencies of 300 and 
4,000 c, the overall slope of the frequency 
characteristic should be “flat” or should rise 
toward the high frequencies at a slope of not 
more than 1 db per octave. An adjustable tone 
control should provide an alternative slope, ris¬ 
ing evenly 6 or 7 db per octave, toward the 
high frequencies. These two extreme adjust- 


and hard-of-hearing ears differ little in their 
tolerable thresholds. 

The instantaneous acoustic output, measured 
as pressure under an earphone at which limit¬ 
ing occurs, should be definitely established, 
either by a semipermanent adjustment, or by 
choice of battery voltages, or by the provision 
of three or four models with significantly dif¬ 
ferent maximum acoustic outputs. 

Of the available devices for limiting the peak 
output level, the simplest is the “peak clipper.” 
Properly adjusted peak clipping protects the 
ear from discomfort and pain while allowing 
a predetermined maximum amplitude of signal 
to be delivered. Compression amplification 
produces less amplitude distortion than simple 
peak clipping, and if an effective compressor 
can be built into a wearable hearing aid it may 


Table 1. The initial and final thresholds of discomfort, tickle, and pain for normal and hard-of-hearing ears 
exposed to speech and pure tones (in db re 0.0002 dyne/cm 2 ). 


Discomfort 

Tickle 

Pain 


Initial Final 

Initial Final 

Initial 

Final 


Normal 


Pure tone 

110 

123 

133 

(> 142) 

140 

(> 142) 

Speech 

117 

130 

128 

135 

138 

(>139) 

Hard-of-Hearing 
Pure tone 

118 

129 

129 

(>141) 

136 

(>141) 

Speech 

121 

130 

129 

134 

135 

(> 137) 


ments are required. At least one intermediate 
setting or a continuously variable tone control 
is recommended. 

Maximum acoustic output should be limited 
so that the ear is protected against transients. 
An investigation by the Central Institute for 
the Deaf 9 determined the thresholds of discom¬ 
fort, of tickle, and of pain produced by pure 
tones and by speech for 46 normal and 46 hard- 
of-hearing ears. The thresholds did not vary 
as a function of frequency between 300 and 
5,600 c. All three thresholds rise systematically 
with successive test sessions, and both the 
initial and final threshold in decibels re 0.0002 
dyne/cm 2 are given in Table 1. (Figures in 
parentheses are the limits set by the apparatus, 
and the median thresholds are above these 
values by uncertain amounts.) These values 
give an approximate indication of the permissi¬ 
ble upper limit. It will be noted that the normal 


provide the ideal means of limiting the maxi¬ 
mum acoustic output. 

The instrument must be sufficiently sensitive 
and free from electrical or other internally 
generated noises to allow it to render intel¬ 
ligible to a normal ear the speech delivered to 
it at a level not more than 10 db above the 
unaided threshold of intelligibility of that same 
normal ear. Patients’ requirements in regard 
to amplification vary so widely that it is prob¬ 
ably desirable to design at least two or perhaps 
three different models of hearing aids. For 
severe hearing losses a maximum gain for 
speech of 80 db should be provided. For mild 
cases, an instrument with a properly designed 
gain control could cover the entire range of 
desired amplification, but it might be unneces¬ 
sarily large, heavy, and expensive for the 
patient who never requires more than 30 or 40 
db of acoustic gain. The justification for more 







WORK DONE AT THE CENTRAL INSTITUTE FOR THE DEAF 


231 


than one model is economy of size and weight 
and expense, and such considerations may 
properly determine the maximum gain to be 
provided by medium-gain and low-gain models. 
No instrument should, however, have less than 
30 db maximum acoustic gain. 

The instrument must be provided with an 
effective gain control, either continuously vari¬ 
able or with numerous intermediate positions 
between maximum and minimum. This range 
should be at least 40 db. The gain control must 
be designed to avoid accidental shift of setting 
by contact with clothing, and it should operate 
roughly linearly on a decibel scale. 

There should be no electric feedback (squeal) 
when the instrument is used at maximum gain 
setting. Acoustic feedback should be dependent 
on the fit of the earpiece and not on the leakage 
from the receiver itself. 

According to the experimental evidence, such 
an instrument would provide the maximum pos¬ 
sible assistance to all those patients for whom 
hearing aids are prescribed. 


i5.5 WORK DONE AT THE CENTRAL 
INSTITUTE FOR THE DEAF 

The Central Institute for the Deaf, St. Louis, 
Mo., participated in the Hearing Aid Project 
under Contract OEMsr-1201. Its chief contri¬ 
butions were: 

1. An exhaustive study of the thresholds of 
tolerance, discomfort, tickle, and pain. 

2. The assembly and installation at Army 
and Navy Aural Rehabilitation Centers of 
electro-acoustic equipment for clinical auditory 
tests. 


15 S1 Tests of Tolerance 

The thresholds of discomfort, of tickle, and 
of pain produced by pure tones and by speech 
were determined in over 11,000 observations, 
on 46 normal and 46 hard-of-hearing ears. 

The initial thresholds for pure tones of pain 
and of tickle lie at about 140 db and 133 db, 
respectively, for all frequencies from 250 to 
5,600 c. The median intensities are greater for 


the normal group than for the hard-of-hearing 
group, owing to the presence of more “tender” 
hard-of-hearing individuals reporting tickle 
and sometimes pain between 120 db and 130 db. 

The threshold of discomfort approximates an 
equal-loudness contour and shows a broad 
minimum (3 db below the mean) between 1,400 
and 4,000 c. The initial median threshold for 
the normal (110 db) lies below the threshold 
for the hard-of-hearing (120 db), but there 
is great dispersion, particularly among the 
hard-of-hearing. 

Similar determinations made with carefully 
monitored (recorded) speech gave a similar 
set of values (see Section 15.4). Tickle is a more 
constant phenomenon for speech than for pure 
tones, and has a lower threshold as measured 
by a VU meter; but pure tones cause discom¬ 
fort at lower threshold in normal ears than 
does speech. 

All three thresholds rise systematically and 
significantly with successive test sessions either 
daily or weekly, and approach limiting values 
after several test sessions. The initial and the 
final values for the various thresholds have 
been presented in Section 15.4, Table 1. 

The increased tolerance is largely but not 
entirely retained after an interval of a week, 
and about half of the increase is retained for 
at least 20 weeks without additional exposures. 

Development of tolerance for speech elevates 
slightly the tolerance thresholds for pure tones. 
Pure tones are less effective in elevating the 
tolerance thresholds for speech. Development 
of tolerance for speech or for pure tones in one 
ear does not increase the corresponding toler¬ 
ance of the other ear of the same individual. 

Tolerance may be developed effectively by 
exposure to loud speech at a level just below 
the threshold of discomfort for several minutes 
a day on three or four successive days. 


15.5.2 Electro-Acoustic Equipment 

Electro-acoustic equipment for clinical audi¬ 
tory tests was installed at Borden General Hos¬ 
pital, Chickasha, Oklahoma, at Hoff General 
Hospital, Santa Barbara, California, and at the 
U. S. Naval Hospital, Philadelphia, Pennsyl- 



232 


HEARING AIDS 


vania. The details of the equipment in the 
several installations varied slightly as improve¬ 
ments were introduced on the basis of experi¬ 
ence and the increasing recognition of the 
importance of free-field tests based on recorded 
or live speech. 

All assemblies were basically two-channel 
systems of high fidelity, one to deliver the test 
material to the subject in a suitably sound- 
treated test chamber and the other a moni¬ 
toring channel for measuring sound levels in 
the chamber and for talkback from the subject. 
Oscillators, turntables, microphones, and spe¬ 
cial noise generators served as sources for the 
test sounds. An amplifier provided the test 
channel with 120 db of gain with undistorted 
power output. A loudspeaker with a frequency 
response reasonably flat from 100 to 9,000 c 
delivered a maximum output of 115 db at a 
distance of 2 m on its axis. Appropriate pro¬ 
vision was made for mixing various input ma¬ 
terials and regulating their intensities accu¬ 
rately at known levels. 

With these installations it was possible to 
conduct all the various tests, except routine 
clinical audiometry, suggested by other sections 
of the Hearing Aid Project, both for assessing 


the impairment of hearing and for the selection 
of hearing aids. The possible tests included: 

A. Monaural tests of the patient without a 
hearing aid. The apparatus is used essentially 
as an audiometer. 

1. Thresholds for pure tones. 

2. Thresholds for speech. 

3. Tolerance, i.e., pain or discomfort 
thresholds, for pure tones. 

4. Tolerance for speech. 

5. Articulation tests with suitable word 
lists. 

B. Free-field binaural tests without a hear¬ 
ing aid. The same tests listed for monaural 
study may be performed under binaural free- 
field conditions. 

C. Free-field tests with hearing aid. 

1. Pure-tone aided thresholds. 

2. Aided threshold for speech. 

3. Tolerance for loud speech with hearing 
aid. 

4. Articulation tests. 

5. Judgments of quality. 

6. Masking and annoyance effects of 
various background noises on speech 
thresholds or articulation scores. 



Chapter 16 

SONIC POSITIONING DEVICES AND DIRECTION FINDING 


T he use of sound signals for direction find¬ 
ing and positioning for military purposes 
has been suggested many times. These sugges¬ 
tions were apparently mostly prompted by 
reference to the everyday experience of easy 
and successful aural localization rather than by 
considerations of the physical problems in¬ 
volved. In reality, the problem is a difficult one 
which admits of practicable solutions only in 
certain special cases and only after considerable 
difficulties have been overcome. 

Since World War I underwater sonic direc¬ 
tion-finding devices have been used with suc¬ 
cess. The following discussion deals with 
certain problems of sonic direction finding in 
air. The nature of the subject matter and the 
relative scarcity of relevant information will 
justify the inclusion of some material which 
may be of general interest but is not directly 
related to the final solution. 


161 SONIC DIRECTION FINDING FOR 
AIRCRAFT INTERCEPTION 

One of the first problems submitted to the 
Electro-Acoustic Laboratory by the Services 
was the investigation of the possibility of 
using an acoustic direction-finding device for 
blind interception of aircraft. The device would 
be used by an attacking plane which has been 
able, by other means of detection, to take up 
a position a relatively short distance behind the 
enemy plane. The device would then enable 
the plane to close in on the enemy and down it 
by gunfire. 

A survey of the problem was made and the 
(largely negative) findings were published 
along with exploratory experimental data in a 
report form. 1 The method of attack proposed 
therein is the use of a highly directional array 
of microphones suitably mounted on the inter¬ 
ceptor. The acoustic energy generated by the 
propellers, engine exhausts, etc., of the target 
plane and radiated toward the rear is picked 
up by the microphone array. The electric signal 


is presented to the attacking pilot in the form 
of a suitably directional visual indication giving 
his position relative to the source of sound on 
the enemy plane. Clearly, it is all-important to 
secure a signal-to-noise ratio at the receiving 
microphone array high enough for reliable 
operation of the indicating device. The report 
mentioned above concludes that this is not 
possible, mainly for the following reasons. 

1. Propellers radiate very little sound in a 
direction opposite to the direction of flight, 
especially at medium and high frequencies. 

2. The noise at the microphone location due 
to the interceptor’s propellers and air turbu¬ 
lence near the microphones is predominant. 

3. The design of a microphone capable of 
operation in high wind velocities and suitable 
for mounting on aircraft is a problem of con¬ 
siderable difficulty. 


16 2 SONIC POSITIONING DEVICE FOR 
USE ON TOWED GLIDERS 

For obvious reasons, it is necessary for towed 
gliders to maintain in flight, within certain 
limits, a predetermined position relative to the 
tow plane. When visibility is good, the glider 
pilot is able to accomplish this simply by watch¬ 
ing the tow plane and piloting accordingly. But 
under conditions of poor visibility, some other 
means must be at his disposal to determine his 
position correctly. 

Of the many possible approaches, only 
acoustic methods will be considered here. Work 
on a sonic positioning indicator [SPI] was 
undertaken by the Electro-Acoustic Laboratory 
in 1942 at the request of the Army Air Forces. 
The project resulted in the development of a 
lightweight model suitable for serving as pre- 
production sample, which was flight tested late 
in 1943. In the opinion of the witnessing labora¬ 
tory personnel, the tests were successful, 
although a series of minor revisions appeared 
to be necessary. These tests are described in 
detail, along with a discussion of the theory 


233 


234 


SONIC POSITIONING DEVICES AND DIRECTION FINDING 


and operation of the SPI, in a report issued in 
February 1944. 2 Several other methods of 
attack are also discussed there. 

From comparison with the problem discussed 
in Section 16.1, it appears that success was in 
no small measure due to four circumstances 
inherent in the problem. First, the tow plane 
is friendly and a sound source can be installed 
to provide adequate acoustic signal level at 
optimum frequency at the glider. Second, there 
is no interfering propeller noise at the glider; 
hence a wider choice of microphone locations 
and more favorable signal-to-noise ratios are 
possible. Third, the cruising airspeeds of towed 



Figure 1 . Location of microphones in glider 
nose. 


gliders are at present rarely, if ever, above 200 
mph. Hence, design of a microphone capable 
of operation at such low airspeeds seemed 
feasible. Fourth, the length of the tow rope is 
only about 300-400 ft. 

The SPI operates as follows: A source of 
sound is mounted on the tow plane such that 
most of its radiated acoustic energy is directed 
backward toward the glider. Three micro¬ 
phones mounted on the nose of the glider, as 
shown in Figure 1, convert some of this acoustic 
energy into electric energy. The electric signal 
is transmitted to the sonic receiver [SR] 
mounted in the glider which furnishes suitable 
visual indications to the glider pilot. The 
microphones are positioned in such a way that 
when the glider is directly behind the tow 
plane sound leaving the source on the tow 


plane at a particular instant strikes the three 
microphones simultaneously, and the acoustic 
signals reaching the three microphones are in 
phase. 

If, however, the glider moves to some other 
position not directly behind the tow plane, the 
signal, in general, no longer reaches the three 
microphones at the same instant and hence the 
three signals are no longer in phase. This is 
illustrated in Figure 2 in which, for reasons of 
simplicity, only two microphones are shown. 
It is evident that the signal reaching micro¬ 
phone M 2 travels a distance farther than the 
signal reaching M 3 . The phase difference be¬ 
tween the two signals is 

^ = 2tt| (1) 

where <f> h is the phase difference, in radians, 
due to an angle © ; , measured in a horizontal 



Figure 2. Showing the phase difference between 
the signals reaching microphones M\ and Mo. 


plane and X is the wavelength of the sound 
signal. But 


x = d sin Q h , (2) 

where d is the separation of M 2 and M 3 . Hence 


2tt d . _ 

4>h = -y sin 0 A . 


(3) 


Since for most angles of interest, sin 0, ( Q h , 


<t> h 



(4) 


Similarly, for an angular displacement of the 
glider measured in a vertical plane the phase 



















POSITIONING DEVICE FOR USE ON TOWED GLIDERS 


235 


difference <f> v between the signals reaching 
microphones M l and M 2 is given by 


Hence <t> r is a direct measure of the glider 
position in a vertical plane (above or below the 
tow plane), while <f> h is a measure of the glider 
position in a horizontal plane (to the left or 
right of the tow plane). It is the function of 
the SR, the electronic equipment to which the 
microphones are connected, to convert these 




Figure 3. Indication of glider position with re¬ 
spect to tow plane. 

two phase differences into readings of a meter 
by means of phase-detecting circuits. The meter 
is of the crossed-pointer type which indicates 
to the glider pilot his actual position relative 
to the desired one. If the glider is, say, above 
and to the right of the correct position, the 
meter needles take the position shown in 
Figure 3. 

It has been tacitly assumed so far that the 
correct flying position for the glider is directly 
behind the tow plane. In reality, glider pilots 
prefer to fly somewhat above the tow plane for 
single tows. In case of multiple tows, they 
must fly at appreciable values of the angle 0 A . 
Controls, therefore, are provided to preset the 
meter zero accordingly. 

16-21 Aerodynamic Noise at Type CG-4A 
Gliders 

The choice of microphone position at the 
glider and the choice of the operating frequency 
of 4,000 c for the SPI was a result of a thorough 
study of the noise produced by air turbulence 


at various positions at or near the surface of 
Type CG-4A gliders. A number of calibrated 
experimental microphones in streamlined hous¬ 
ings were mounted at various positions on the 
glider. The noise levels at these microphones 
were measured for various airspeeds by means 
of a sound analyzer in bands 200 c wide. 

Typical values of noise levels for a micro¬ 
phone mounted on the glider nose are given 
in Table 1 as a function of frequency. 


Table 1. Ambient noise levels at nose of CG-4A 
glider. (Bandwidth 200 c; indicated airspeed 135 
mph; altitude 5,700 ft.) 


Frequency 
(cycles per second) 

Noise level 

(db re 0.0002 dyne/cm 2 ) 

200 

120.5 

400 

112 

600 

109.5 

800 

110.5 

1,000 

107 

1,400 

93 

1,800 

81.5 

2,200 

77.5 

2,600 

75.5 

2,800 

75 

3,200 

73 

3,600 

70.5 

4,000 

71 


Inspection of the values in Table 1 shows 
that the low-frequency noise is predominant. 
The levels are comparatively low at frequencies 
beyond about 2,000 c. It is in this frequency 
range, then, that the transmission frequency 
should be located for favorable signal-to-noise 
ratios. This assumes, of course, the feasibility 
of producing a given signal level at the glider. 
Some of these questions will be taken up later. 

From inspection of similar data for different 
microphone locations it appeared that at low 
frequencies the noise levels are greatest at the 
nose of the glider and least out on the wings 
at locations sufficiently removed from the wing 
profile. At frequencies above about 2,000 c, 
however, the reverse is true in some instances. 
For this and reasons of practicability, the glider 
nose has been chosen as microphone location. 

16.2.2 Transmission of Acoustic Signals 
from Tow Plane to Glider 

When the tow plane and glider are moving 
through the air, the air through which the 














236 


SONIC POSITIONING DEVICES AND DIRECTION FINDING 


sound signal must pass is in a turbulent state. 
As a result of this and other factors, the den¬ 
sity of the air through which the signal is 
transmitted varies irregularly with both time 



Figure 4. Wind-driven siren. 


and position. Hence, the signal reaching the 
microphones does not have a constant level, but 
fluctuates sometimes by large amounts. 


15 db at 2,000 c, 20 db at 4,000 c, and 25 db 
at about 7,000 c. 

Turbulence of the air intervening between 
tow plane and glider causes distortion of the 
wave fronts of the progressive sound wave 
emitted by the sound source on the tow plane. 
This would naturally affect the operation of the 
sonic receiver. No direct measurements of these 
distortions of the wave front reaching the 
glider were made. Preliminary flight tests of 
an SPI showed, however, that reliable opera¬ 
tion of the SR and phase-discriminating cir¬ 
cuits could be maintained, provided a favorable 
average signal-to-noise ratio could be obtained 
at the glider. This could be realized by using 
a sound source which was capable of generat¬ 
ing a sound level at the glider of about 85 db 
(re 0.0002 dyne/cm 2 ) at a frequency of 4,000 c. 
Band-pass filters of 200-c bandwidth were used 
in each of the three microphone channels. 

This choice of signal frequency was a com¬ 
promise between favorable signal-to-noise ratio 



Figure 5. Sound source mounted on tow plane. (Army Air Forces photo.) 


The following values of rapid (about 10 msec 
minimum duration) fluctuations of the signal 
were recorded in flight using a certain experi¬ 
mental sound source: about 10 db at 500 c, 


and practicable physical dimensions and power 
requirements of the sound source on one hand 
and signal fluctuations at the glider and atmos¬ 
pheric scattering and absorption on the other. 








POSITIONING DEVICE FOR USE ON TOWED GLIDERS 


237 


16.2.3 rpi n i n 

Ine bound bource 

It consists of a wind-driven siren mounted 
on the tow plane and aimed toward the glider 



Figure 6. Siren with rotor detached. 

(see Figures 4 and 5). The siren was adopted 
after thorough trials of a great number of 
other devices, ranging from organ pipes to 
modulated air-flow loudspeakers. 2 ® 

Some of the details of the construction of the 
siren may be gleaned from Figures 6 and 7. 


the other end of the cylinder these passages 
are alternately opened and closed by a slotted 
rotor disk, whose slots correspond to the slots 
in the housing. It is driven by the air flow at 
10,000 rpm, interrupting it 4,000 times per 
second. For frequency-control purposes, a small 
electric control motor with a centrifugally 
operated switch is provided on the shaft. This 
control motor is essential to keep the fre¬ 
quency of the acoustic signal sufficiently con¬ 
stant to fall within the pass band of the filters 
in the SR. The shape of the cross section of 
the rotor blades and the spacing between rotor 
and stator are critical design parameters for 
a given model. The width of the radiation beam 
of the siren is about ± 20 degrees for points 
6 db down from the value on the axis. The total 
weight of the sonic equipment installed in the 
tow plane can be kept below about 11 lb with¬ 
out difficulty. 

16.2.4 The Sonic Receiver 

A block diagram of the SR is shown in 
Figure 8. The three microphones supply the 



The cylindrical housing encloses 24 radial fans, electric signals to three identical channels, con- 
They form long passages through which the sisting of band-pass filters and amplifiers with 
air entering at one end is forced in flight. At suitable automatic volume control. The phase 






























































238 


SONIC POSITIONING DEVICES AND DIRECTION FINDING 


CHANNEL A 



FILTERS A VC AMPLIFIERS 


Figure 8. Diagram of sonic receiver. 



Figure 9. Sonic receiver. 



Figure 10. Proposed location of sonic receiver in CG-4A glider. (Army Air Forces photo.) 



































POSITIONING DEVICE FOR USE ON TOWED GLIDERS 


239 


differences between the microphones M 1 and M 2 , 
and M 3 and M„, respectively, are transformed 
into deflections of the indicating meter by two 
phase-detecting circuits of conventional type. 

The preproduction model of the SR is shown 
in Figure 9. It consists of two units: the control 
box (containing the position-indicating meter 


of the glider. The microphones are operated 
electrically in an “off-resonance” condition 
which tends to minimize matching difficulties 
and change in microphone phase characteris¬ 
tics, resulting from changes in the conditions 
of ambient temperature, shocks, etc. A sample 
frequency-response curve is plotted in Figure 



INCHES 


Figure 11. Microphone unit used on glider. 

and the filters) and the amplifier box (contain¬ 
ing the amplifiers and the power-supply dyna- 
motor). The two units are connected by means 
of the shielded cable shown in the figure. Figure 
10 shows the proposed mounting place for the 
control box for the SR near the pilot’s position 
in a CG-4A type glider. 



Figure 12. Location of microphones on CG-4A 
glider. (Army Air Forces photo.) 



IOOO 2000 3000 4000 5000 


FREQUENCY IN CYCLES PER SECOND 


Figure 13. Frequency-response characteristic of 
microphones. 


13. Graphs indicating meter current versus 
position angle are shown in Figure 14 for 
input signals of varying quality. These tests 
were made in the laboratory under simulated 
flight conditions. 213 The accuracy of the device 



Figure 14. Showing meter current vs position 
angle for signals of varying quality. 


One of the three microphones is shown in 
Figure 11. They are modified Western Electric 
Type MC-253 magnetic units. The design of 
the case has been chosen so that the micro¬ 
phones may be mounted in holes in the glider 
nose (see Figure 12) flush with the surface 


seems to be satisfactory for practical purposes. 

The entire equipment installed in the glider 
weighs about 30 lb. The equipment draws about 
4.5 amp from the 12-v glider battery. 

The device was not adopted by the U. S. 
Army Air Forces. 















































































Chapter 17 

SONIC TRUE AIRSPEED INDICATOR 


171 INTRODUCTION 

T he work reported in this chapter was 
begun early in 1944 by the Electro-Acoustic 
Laboratory at the request of the Navy. A letter 
from the Assistant Coordinator of Research 
and Development to the National Defense Re¬ 
search Committee [NDRC] reads, in part, as 
follows: “The Bureau of Aeronautics desires 
the development of a sonic true airspeed indi¬ 
cator. This device . . . depends on the measure¬ 
ment of the difference in time taken by a sound 
impulse to travel between two points on the 
aircraft, when traveling with and against the 
relative air flow.” 

Airspeed indicators developed in the past 
depend for their operation on a differential 
pressure effect, the cooling effect of an air 
stream or the deflection of a rotatable member. 
These indicators include the pitot-static system, 
the Venturi tube, the hot wire anemometer, the 
deflecting plate, the cup anemometer, and the 
vane anemometer. A survey of the conventional 
types of airspeed-measuring devices may be 
found in a report. 2 

At the present time, the pitot-static system 
is by far the most widely used type of airspeed 
indicator. This device measures the difference 
between the pressure developed by the impact 
of the air stream on the front of a tube (pitot 
pressure) and the pressure in a certain region 
where stable air flow exists (static pressure). 
Since the impact pressure is proportional to the 
air density, which, in turn, is a function of 
altitude and temperature, the pitot-static indi¬ 
cator does not yield true airspeed directly. In 
general, corrections have to be applied for 
temperature and ambient static pressure to 
obtain true airspeed indications. These correc¬ 
tions can be made manually, point by point, 
or with the aid of specially designed computers 
in a continuous manner. 

The need for accurate and continuous true 
airspeed indications arises primarily in connec¬ 
tion with ground-position indicating devices 


and devices measuring the distance traveled 
relative to the air, for use in fast high-altitude, 
long-range aircraft. True airspeed is defined 
as the speed of the aircraft relative to the 
air at a large distance. Hence it is evident that 
all airspeed indicators are subject to the so- 
called installation error (in addition to any 
other errors), since they measure the speed of 
the aircraft relative to the air in its vicinity. 
This air is, in general, not stationary with re¬ 
spect to the air at a large distance because of 
the disturbances set up in flight. 

Some of the merits and demerits of the pitot- 
static system can be stated, in brief, as follows: 

1. Corrections for temperature and pressure 
must be applied. Special computing mechanisms 
are necessary. Accurate and meaningful meas¬ 
urements of temperature and pressure are 
difficult in many instances. 

2. Pitot-static indicators begin to fail when 
the stream velocity reaches about eight-tenths 
of the local sonic velocity. They cannot be used 
at supersonic speeds. 

3. The overall error of pitot-static indicators 
is least at medium airspeeds. It becomes larger 
both at low and at high stream velocities. 

4. The pitot tube and its mountings disturb 
the air stream by their presence. 

5. The pitot tube is simple, light, rugged, 
and easy to duplicate. 

On the other hand, the possible advantages 
of a sonic true airspeed indicator [STASI] 
which one might hope to realize are: 

1. Except for installation error, true air¬ 
speed is measured. 

2. Since the velocity of sound is virtually 
independent of pressure, the indications of the 
STASI are independent of altitude. 

3. It is possible to arrange the acoustic sys¬ 
tem of the STASI in such a way that the indi¬ 
cations are also independent of temperature. 

4. On aircraft and in wind tunnels, the sound 
source and microphones can be mounted flush in 
an airfoil or wind tunnel wall. Hence stream 
disturbances are practically nonexistent. 


240 


SOUND PROPAGATION IN MOVING AIR 


241 


5. Operation is possible at high airspeeds, 
including operation at supersonic speeds. 

6. Continuous true airspeed indications can 
be secured with comparative ease. 

On the other hand, the working accuracy is 
as yet undetermined and may be larger than 
for the pitot-static system. The STASI is of 
necessity more complex than the pitot-static 
system. 

At the present stage of the art the following 
applications for a STASI can be listed, in the 
order in which they are feasible and realizable 
at the present time: 

1. Continuous true airspeed indications at 
low airspeeds (order of 100 mph). Present 
pitot-static installations on lighter-than-air 
ships are unsatisfactory and can with advan¬ 
tage be replaced by STASI installations. 

2. Calibration of wind tunnels. A method 
has been worked out by which the velocity 
distribution in a wind tunnel can be determined 
by sonic means without appreciably disturbing 
the stream. 


17 2 GENERAL THEORY OF OPERATION 

The measurement of airspeed by sonic meth¬ 
ods depends on the fact that the transit time 
of a sound wave between two fixed points in 
an air stream is a function of the airspeed in 
the region between the points. Analytical ex¬ 
pressions of the relation between transit time 
and airspeed can easily be obtained for the 
case of constant sonic velocity and airspeed. 4 
It is assumed here that the acoustic signal is 
a continuous wave having a single frequency. 
This offers certain practical advantages over 
devices using sound pulses. 

Two simple arrangements of the basic 
sonic array which consists of a sound source 
( S) and two microphones or receivers (Ri, R 2 ) 
suggest themselves: one where R u S, and R 2 
are aligned in the order given and the other 
where the sound source and one of the micro¬ 
phones are interchanged. Measurements of the 
phase angles between the electric signals gen¬ 
erated by the microphones or of the phase 
angles between the microphone signals and the 
electric input to the source yield the desired 


data for computation of the component of true 
airspeed in the direction of the line of the 
array. In Table 1 a summary of the operational 
characteristics of sonic arrays is given. From 
inspection of frequency and temperature errors 
it can be seen that a (symmetrical) array can 
be built which is free from temperature error. 
It is this type of array which was flight-tested 
in a Navy airship as described in Section 17.4. 


17 3 EXPERIMENTAL STUDY OF SOUND 
PROPAGATION IN MOVING AIR 

Wind Tunnel Studies at Low 
Airspeeds 

In the discussion of the preceding section 
it was tacitly assumed that the acoustic arrays 
in their practical embodiments were mounted 
flush in an airfoil or some other surface bound¬ 
ing the flow. It becomes all-important, there¬ 
fore, to study the propagation of sound along 
boundaries of an air stream, especially at high 
stream velocities. There is no a priori assurance 
that a sound signal would propagate along 
such boundaries as it would in free space. 
Normal propagation was assumed in deriving 
the expressions in Table 1. 

In order to explore the conditions existing 
in the boundary layer an extensive test pro¬ 
gram was undertaken at the Electro-Acoustic 
Laboratory and at the Gordon McKay Labora¬ 
tory of the Graduate School of Engineering 
at Harvard University. 

As a first step, an acoustic array of the 
asymmetrical type was tested, mounted in the 
floor of the Harvard low-speed wind tunnel. 
A conventional electronic phasemeter was used 
to determine the phase differences at an operat¬ 
ing frequency of 4,000 c. The microphones and 
the sound source were mounted in a stream¬ 
lined wooden “fish” which projected up from 
the tunnel floor about 2 in. A housing of sound¬ 
absorbing material was placed over the array 
to minimize troublesome acoustic reflections. 
The airspeeds as computed from the measure¬ 
ments of phase angle were compared with the 
values obtained from pitot-static measure- 



242 


SONIC TRUE AIRSPEED INDICATOR 


Table 1. Summary of operational characteristics of various sonic airspeed measuring arrays. 


Array 

Airspeed 

Quantity 

Measured 

Frequency 

Error 

Temperature Error 


A 

V 

© 1 
d 

© :: 

d 


a - i) 

none 

none 


&rk) 

v meas ^ 1 

none 

© * 

Vtrue f 

same as 

above 

»=£ 0-c© 

t 

none 

v meas. _i. T c / v^, eas / T 
v true ' T V I' T 0 /y 


4> 

v meas. p 

v+rue T 

same as above 



, ®| 

I or d 

' foV 

v = f~ c 

t 

none 

v meas~ v true ~ 

V 

© 

2TT fd 

v = , c 


v meos.— v true = 

r 

(c +v meas.)(l — 

same as above 




c Sonic velocity at temperature T ti Transit time of signal between source S 

co Sonic velocity at temperature To i| nc * m i cro Ph°ne Ri (i — 1,2) 

<Pi Phase angle between the source voltage 

d Separation between microphone and source and the microphone output (i = 1,2) 

v Velocity of air stream <P = <t >2 — <t> l Phase angle between the two microphone 

outputs 

/ Frequency of signal T Absolute temperature 

































SOUND PROPAGATION IN MOVING AIR 


243 


ments. Table 2 gives the results of this com¬ 
parison. 


Table 2. Results of airspeed measurements in a 
low-speed wind tunnel by the sonic method. 


V Sonic 
(mph) 

V Pitot 
(mph) 

Difference 
(per cent) 

62.1 

58.8 

+ 5.5 

85.3 

86.2 

—1.2 

101.2 

104.0 

—2.7 

111.1 

120.0 

—7.4 

122.3 

134.0 

—8.7 


It was concluded that (a) the STASI is a 
workable device at low airspeeds, (b) there are 
indications of systematic deviations of the air- 


Thus the flow can be observed optically by the 
striation, or schlieren, method. Briefly, this 
method is a way of making visible variations 
in the index of refraction of the air in the 
tunnel. In the present case, these variations 
are produced by an acoustic compression wave 
generated by the discharge of a spark gap 
located in the air stream. By photographing 
these compression waves at successive known 
time intervals by means of a brilliant light 
flash of extremely short duration successive 
snapshots of the compression wave are ob¬ 
tained. From the shape and structure of these 
wave fronts, conclusions can be drawn with 


® 

SPHERICAL 

MIRROR 



speed obtained by the sonic method from the 
value obtained by the pitot-static method. These 
deviations tend to increase with airspeed. 


17 ' 3 ' 2 Studies in a High-Speed Wind 
Tunnel: Methods and Apparatus 

In order to explore the nature of these devia¬ 
tions further and to attack the whole problem 
from a broader angle, an experimental study 
of sound propagation in moving air and an 
investigation of the theory of such propagation 
was initiated, using the Harvard high-speed 
wind tunnel. This tunnel is of the suction type 
with one end open to the atmosphere. Mach 
numbers a (M) as high as 1.8 can be reached. 
The test section is 1% in. wide and has a 
height variable between 4 and 6 in. Optically 
flat glass sidewalls 8 in. square can be placed 
anywhere along a 2-ft section of the tunnel. 

a The Mach number M is defined here as the ratio of 
the stream velocity to the local sonic velocity. 


reference to the propagation of sound waves 
in moving air. 

Of the many possible arrangements of the 
optical elements in a striation apparatus the 
so-called coincidence system was used. Such 
a system is shown schematically in Figure 1. 
The light source is a small, sharp-edged mirror 
M upon which an image of the source of 
light L is formed by lens N. M is located near 
the center of curvature of the large spherical 
mirror M' but slightly off the axis. The subject 
plane S of interest is immediately in front of 
M'. After traversing the subject plane twice 
the light is partially intercepted by the knife 
edge B located at the conjugate point opposite 
the source image on mirror M. A camera or 
ground-glass screen can be placed at P to allow 
photographic or visual observation. This sys¬ 
tem can be made free of all optical aberrations 
with relative ease. 

If the air in the subject plane S is of uniform 
index of refraction, the field of view at P will 
be uniformly illuminated. If, however, there 





















244 


SONIC TRUE AIRSPEED INDICATOR 


is a local change in index of refraction (schliere 
or striation) in the subject plane, clue, say, 
to a sound wave, a change in the illumination 
at the corresponding position in the image 
plane P will be apparent. Note that this 
arrangement is sensitive only to striation caus¬ 
ing deflections of light perpendicular to the 
knife edge. 


In the experiments to be described, the wind 
tunnel with its glass walls was located in the 
subject plane S' as indicated in Figure 2. To 
obtain snapshots of sound waves generated by 
sparks in the wind tunnel electronic equipment 
is used, a block diagram of which is shown in 
Figure 3. The chain of operations is as follows: 
A push button initiates the discharge of the 


DIRECTION OF FLOW 



Figure 2. Striation system with high-speed wind tunnel. 


The double passage of light through the 
striae is advantageous in that it doubles the 
sensitivity of the system for small deflections 
of the light beam. In cases where this no longer 
holds, the incident and reflected beams no 
longer traverse the striae at the same portion, 



Figure 3. Block diagram of electronic equip¬ 
ment for high-speed photography. 


and a double image results. Successful opera¬ 
tion of the coincidence system depends, among 
other things, on locating the subject plane as 
closely as possible to the spherical mirror M' 
and avoiding the phenomenon of split images 
discussed above. 


spark-gap sound source located in the wind 
tunnel, in which the desired conditions of 
flow have been set up. The adjustable time 
delay circuit is synchronized with the start 
of the sound wave by means of the electro¬ 
magnetic radiation from the spark-gap wires. 
The delayed pulse output of the delay circuit 
triggers the switching circuit which in turn 
flashes the high-speed light source, supplied by 
the high-voltage power supply. This source 
furnishes the light to the optical striation sys¬ 
tem and a picture of the sound wave is obtained 
after a known time interval has elapsed from 
the instant the spark was fired. 

The spark-gap sound source consists essen¬ 
tially of a power supply unit, a Ford spark 
coil and the spark gap itself. Its electrodes 
are made of pure tungsten wire 0.030 in. in 
diameter, ground to pin-point ends. The inten¬ 
sity of the sound wave from the spark depends 







































































































SOUND PROPAGATION IN MOVING AIR 


245 


on the voltage and capacitance across the gap, 
the shape of the electrodes and their separation. 
The quantitative relationship existing between 
these variables is as yet unknown. In the region 
near the gap the sound waves have abnormally 
high velocities of propagation. It has been 
found, for example, that the sound wave from 
a 5-mm gap (about the largest used) reaches 
normal velocity of propagation 2.3 cm from 
the gap. 

The time delay circuit is designed to permit 
time delay settings ranging from a few micro¬ 
seconds to about 300 psec. The core of the device 



Figure 4. Flash light source (GE Type H-6). 

is an arrangement of two tubes in a trigger 
circuit which has come to be known as the “flip- 
flop” circuit of the positive grid type. It is 
closely related to the Eccles-Jordan trigger cir¬ 
cuit except that it has only one position of stable 
equilibrium. With suitable precautions and 
careful design it is possible to obtain a linear 
relationship between the settings of a capacitor 
and the time delay between the input and 
output pulses of the device. The input pulse is 
the pulse of electromagnetic radiation from the 
spark gap. The output pulse is a surge gen¬ 
erated by the trigger circuit returning to its 
stable equilibrium. 

This output pulse is passed on to the switch¬ 
ing circuit which consists essentially of an 
amplifier and a thyratron switch. When the 
thyratron grid is driven positive, the high 
voltage on the plate causes it to fire and the 
high-voltage power supply discharges through 
the light source, producing a brilliant flash of 
less than one-microsecond duration. Thyratrons 
of the FG-17 and 4C35 types have been used 
with success. 

The General Electric Type H-6 lamp is suit¬ 


able as either a continuous or a flash light 
source for schlieren systems. This lamp is a 
quartz capillary tube 3% in. in length and 
^4 in* in outside diameter with mercury pool 
electrodes (see Figure 4). For operation as a 
flash source of light a voltage of 5 to 6 kv 
is necessary. The light intensity is comparable 
with the intensity of a carbon arc. 

A series of photographs of a sound wave 
in still air taken for a number of time delays 
increasing in succession is shown in Figure 5. b 
For comparison, a gauge block 5.00 cm wide 
has been included in the photographs. Such a 
set of photographs can be used to calibrate 
the electronic equipment directly in terms of 
increment of capacitance (in the trigger cir¬ 
cuit) per microsecond difference in time delay. 
The capacitance values C corresponding to the 
different time delays t were determined by a 
precision capacitance bridge. The correspond¬ 
ing radii of the sound waves were carefully 
measured by comparison with the image of 
the gauge block, using a traveling microscope. 
From a measurement of the temperature the 
sonic velocity can be computed 3 and hence the 
overall calibration can be determined. Five 
determinations of the constant of proportion¬ 
ality between C and t over a period of several 
months yielded the value 29.25 psec per 0.001 
pf ± 0.4 per cent. From other observations 
it was also ascertained that, for all practical 
purposes, the time vs capacitance line passes 
through the origin. 

Figure 6 shows the set of experimental points 
corresponding to the photographs shown in 
Figure 5. The slope of the curve at any point 
is the sonic velocity at that distance from the 
spark gap. For example, at a distance of 0.55 
cm from the source the wave in Figure 5 is 
traveling with a velocity nearly 50 per cent 
larger than the normal value. 

1733 Studies in a High-Speed Wind 
Tunnel: Results and Conclusions 

The conclusions formulated in this section 
are based on the results obtained up to Septem- 

b The fact that condensations and rarefactions show 
dark and light on one half of the pictures and reversed 
on the other can easily be explained from the optical 
properties of the schlieren system used. 








246 


SONIC TRUE AIRSPEED INDICATOR 





+ TIME DELAY 
20 p sec 


TIME DELAY 
146 p sec 


4 


S-360 




4»TIME DELAY 
29 p sec 


TIME DELAY4 
205 p sec 



S-362 


4 


TIME DELAY 
44 p sec 

TIME DELAY 
263 p sec 



S-365 



S-367 


Figure 5. Striation photographs of sound waves in still air. 


























SOUND PROPAGATION IN MOVING AIR 


247 


ber 1945 and are to be regarded as tentative. 
Most conclusions are based on a qualitative 
study of a considerable amount of striation 
photographs of sound waves in moving air. 
They are indicative of further problems await¬ 
ing solution. 

In Figure 7 a striation photograph 0 of a 
sound wave is shown propagating in an air 
stream of extreme turbulence, which was 
caused by the particular spark-gap arrange¬ 
ment used. It is evident that the wave front 
has been broken up by turbulence, but it can 
still be located without difficulty. 

Figure 8 shows a sample of photographs 
taken with a sound source consisting of five 
gaps connected electrically in series and 
mounted in a bakelite strip. The bakelite strip 
was mounted flush in the floor of the tunnel. 

From these and other photographs the fol¬ 
lowing conclusions can be drawn. 

1. Fine-scale turbulence, such as is observed 
in the striation photographs, does not seriously 
interfere with the propagation of sound waves 
from spark gaps. As the turbulence increases 



Figure 6. Radius of sound waves as a function 
of time. 


the definition of the wave fronts becomes 
poorer. 

2. At subsonic velocities, sound is propagated 
both upstream and downstream along a flat 
surface over relatively short distances only. 

3. The density gradient produced by the 
sound wave is, in general, higher at a point 
upstream than at the corresponding point 

c Note that in all photographs the air stream is flow¬ 
ing from right to left. 


downstream. The ratio of the downstream 
density gradient to the upstream density 
gradient is (1 — M) 2 /(l + M) 2 , for the 
subsonic case. The upstream wavelength 41 is 



Ms.72 Time DELAY 88jisk. S-39 


Figure 7. Striation photograph of sound waves 

in a turbulent stream. 

shorter than the wavelength downstream. The 
ratio of the two is (1 — M)/(l-f- M). 

The problem of the propagation of acoustic 
shock waves in a boundary layer where a 
velocity gradient exists has been approached 
theoretically. The following conclusions can be 
drawn. 

1. The sound upstream from the sound 
source is refracted away from the surface by 
the boundary layer. This refraction results in 
a significant decrease in the sound level at 
the surface. 

2. The sound downstream of the sound 
source is refracted toward the surface by the 
boundary layer. This refraction results in total 
internal reflection for a sizeable portion of the 
sound waves. The reflected rays return to 
the surface and are reflected back into the 
boundary layer where they again undergo 
total internal reflections. This “trapping” of 
the rays will result in interference phenomena 
for continuous waves. 

It should be noted that, although important 

d The wavelength is taken to be the distance from the 
leading edge of the compression wave to the trailing 
edge of the rarefaction wave. It is of the order of 1 cm 
here. 































248 


SONIC TRUE AIRSPEED INDICATOR 


in their own right, some of the above conclu¬ 
sions are not directly applicable to a practical 
STASI installation using sound waves of a 
wavelength many times as long as the length 
of waves investigated in the wind tunnel. 

The question is not yet settled as to whether 
the turbulence around the source and micro¬ 
phones in a practical installation on an aircraft 
behaves in a manner acoustically similar to that 
of the fine-scale turbulence present in the 
tunnel. As to the propagation of long-wave 


flight-tested. 5 Its feasibility at low airspeeds 
had been demonstrated by the wind tunnel tests 
discussed in Section 17.3.1. 

From Table 1 in that section it is evident 
that, for a STASI using a continuous wave 
signal and a symmetrical array, a measurement 
of the outputs of the two microphones relative 
to the input to the sound source is required 
if the indications are to be independent of the 
temperature. In addition, frequency and micro¬ 
phone separation must be known. 



M =.33 TIME DELAY 58 >i sec S-568 



M = .33 TIME DELAY 205jjsec 



^=.82 TIME DELAY 58 jj sec 


- * 

M = .82 TIME DELAY 205>i sec 


Figure 8. Striation photographs of sound waves from a multiple spark gap in moving air. 


sound along an airfoil in high turbulence, it 
can only be conjectured that if propagation 
takes place it comes about not by direct propa¬ 
gation in the boundary layer, but by spreading 
into the layer of Huyghens wavelets from the 
wave external to the layer. 

DESIGN AND TESTS OF A STASI FOR 
USE ON A NAVY BLIMP 

A STASI was designed based on the prin¬ 
ciples discussed in Section 17.2 and successfully 


The general operation of the experimental 
STASI will now be described. (A detailed dis¬ 
cussion of the circuits can be found else¬ 
where.) 5 ' 1 Since the flight tests were conducted 
for the sole purpose of obtaining experimental 
data no attempt was made to minimize size and 
weight of the electric equipment. Proposed 
modifications desirable for the construction of 
a preproduction model are given at the end of 
this section. 

A block diagram of the laboratory model of 
a STASI is given in Figure 9. An oscillator, 









STASI FOR USE ON A NAVY BLIMP 


249 


operating at a frequency of 4,000 c, provides 
the electric power to drive the sound source 



Figure 9. 
indicator. 


Block diagram of sonic true airspeed 


sound head mounted in a suitable position on 
the airship. From each of the two external 
microphones which receive the airborne sound 
the signal is passed through an amplifier with 
automatic volume control to a phase discrimina¬ 
tor unit. 

The phase angle indicator makes use of a 
Bendix torque unit which contains a reversible 
motor geared to a precision Autosyn with 90- 
degree field windings. The output signal of the 
oscillator is used to feed the two stator wind¬ 
ings of the Autosyn with voltages shifted by 
90 degrees in phase. A rotating circular mag¬ 
netic field is formed by the two stator coils 
and the phase of the voltage induced in the 
rotor winding depends upon the orientation of 
the rotor in the circular field. This voltage is 
fed into the phase discriminator circuit and 
compared with the microphone output voltage. 



Figure 10. Sonic arrays mounted on Navy K airship. (U. S. Navy photo.) 

and to provide the reference voltage for the If the phase angle between these two signals 
measurements of phase angle. The sound source is not a fixed value (90 degrees), an error 
and the two microphones are in an external voltage is generated by the phase detector. This 












































250 


SONIC TRUE AIRSPEED INDICATOR 


voltage actuates the motor in the torque unit 
which turns the rotor of the Autosyn until zero- 
error voltage obtains. In this manner, a pointer 
mounted on the rotor shaft gives a direct and 
linear measure of the phase of the microphone 
output signal with respect to the reference 
voltage. 

The experimental apparatus consists of two 


Each of the arrays contains a dynamic ear¬ 
phone (Permoflux PDR-10) as the sound 
source, flanked by two modified Western Elec¬ 
tric Type MC-253 magnetic microphones. 

Figure 12 shows the projected head in detail. 
A typical frequency-response characteristic of 
the microphones is given in Figure 13. The 
microphones are operated in an “off-resonance” 



Figure 11. Detailed view of sonic arrays mounted on Navy K airship. (U. S. Navy photo.) 


of these phase-measuring channels, one for each 
microphone. The two phase angles <f> t and cf> 2 
were read separately and the airspeed calcu¬ 
lated from these readings. 

Figures 10 and 11 show how two experi¬ 
mental sonic arrays (heads) were mounted on 
a Navy K-24 airship. The projected head ex¬ 
tends 27 in. below the envelope on a streamlined 
strut, while the flush-mounted head can be 
seen cemented directly to the envelope with a 
fabric patch. 


condition which facilitates the matching of the 
two units. 

Figure 14 gives an estimate of the noise from 
aerodynamic and other causes picked up by the 
microphone for certain flight conditions. The 
cross on the graph corresponds to the signal. 
These data show that filters in the microphone 
channels are superfluous under these circum¬ 
stances. 

Figure 15 shows the electronic and indicat¬ 
ing equipment mounted near the navigator’s 









STASI FOR USE ON A NAVY BLIMP 


251 


table. Details of the phase angle dials and 
Bendix torque unit are apparent from the 
photographs of Figure 16. 

The equipment described above was cali¬ 
brated and tested by flying along a straight- 
line ground course in both directions under 



Figure 12. Projected sonic head. 


controlled flight conditions. During each test 
run, record was made of the elapsed time, drift 
angle, airspeed measured by pitot-static means, 
airspeed measured by two Bureau of Standards 
heads (one trailing and one mounted near the 


















































y 










y 











































































1000 4000 10000 

FREQUENCY IN CYCLES PER SECOMO 


Figure 13. Frequency response of microphone. 


sonic head), and the two phase angles of the 
sonic indicator. The time required to fly the 
course of 2.615 nautical miles was determined 
by means of aerial photographs of the ground 
course and a clock. The oscillator frequency 
was checked periodically in flight and the phase 


angles for zero airspeed were measured in the 
hangar prior to the flights. 

From the drift angle, flight time, and course 
length the average “clocked” airspeed can be 
computed. The controlling error in the com¬ 
puted clocked airspeed is the error resulting 
from nonideal flight conditions of fluctuations 
in wind and variation in heading angle. The 
probable error due to these causes is estimated 
at 2 per cent. The readings of the pitot-static 
and standard heads were taken for check 
purposes only. 

Figure 17 shows the average airspeed com¬ 
puted from the indications of the STASI plotted 
against average clocked airspeed. The depar¬ 
ture of the experimental points from the 
straight line of optimum fit is less than 2 per 
cent. The horizontal line through each point is 
an indication of the standard deviation of the 
clocked airspeed for a fixed engine power, and 
the vertical line indicates the standard devia¬ 
tion of the sonic airspeed for a number of runs. 


— ENGINE 1700 MM 

I 5 * ON POR-iO SIGNAL SOURCE 

















S 

sign* 

































J 












/ 


\ 












\ 













\ 







































100 1000 14000 
FREQUENCY IN CYCLES PER 3£C0N0 


Figure 14. Noise analysis of microphone output. 


The dotted line would be obtained if the STASI 
read clocked airspeed directly. The deviation in 
slope (about 5 per cent) between the two lines 
is due to the installation error. The reason for 
the intercept at zero airspeed is not fully under¬ 
stood as yet. 

The foregoing results were obtained with the 
projected head. The array mounted flush with 
the envelope of the airship proved unsatis¬ 
factory. 

By further development the bulk and weight 
of the equipment described above can be ma¬ 
terially reduced. The sonic arrays can be 
simplified if the temperature independence is 









































































252 


SONIC TRUE AIRSPEED INDICATOR 



Figure 15. Experimental STASI indicator installation on Navy blimp. (U. S. Navy photo.) 



Figure 16. Phase angle dials with torque units. 



Figure 17. Plot of average indicated airspeed 
(projected head) vs average clocked airspeed. 











































VELOCITY DISTRIBUTION IN WIND TUNNELS 


253 


sacrificed. The two phase angle indications 
can be combined into a single indication of 
a suitable instrument, calibrated directly in 
miles per hour. This can be done by electrical 
or mechanical means. A mechanical solution 
is given elsewhere. 5b 

17 5 DETERMINATION OF THE VELOCITY 
DISTRIBUTION IN WIND TUNNELS 
BY A SONIC METHOD 

Successive photographs of an acoustic pulse 
wave with various known time delays between 


the region of interest. A series of successive 
snapshots of the sound waves is then taken. 
The slope of the distance vs time plot of the 
wave fronts along the line of flow is equal to 
the velocity of propagation relative to the 
tunnel, namely, the difference between the local 
sonic velocity and the stream velocity. From 
this relation and the Bernoulli equation the two 
unknowns, namely, the stream velocity and the 
sonic velocity (and hence the free stream 
temperature), can be determined. A tempera¬ 
ture measurement of the undisturbed air feed¬ 
ing the tunnel is also required. The estimated 



successive positions of the wave front can be 
used to determine velocities and temperature 
in wind tunnels. Since the spark gaps generat¬ 
ing the sound can be accommodated in many 
cases with a minimum of stream disturbance, 
the sonic method may constitute a useful tool in 
wind tunnel research. 

In the case of parallel (one-dimensional) flow 
a simple procedure can be followed. The spark 
gap is mounted in the tunnel downstream of 


accuracy in computing Mach numbers from 
these data is ±1 per cent. 

Two other methods can be used which are 
applicable to two-dimensional flow. They yield 
the two components of the stream velocity and 
the sonic velocity (and hence the temperature) 
as a function of position in the stream. 

It is assumed that the motion of the acoustic 
disturbance is the same as though at each 
moment each point on it were moving with 































































254 


SONIC TRUE AIRSPEED INDICATOR 


a velocity compounded of (1) the wave velocity 
c at the point in question and (2) the stream 
velocity at that point. 1 With these assumptions 
it can be shown that the following equation 
(wave-front equation) holds: 

where x and y are the two coordinates, u and 
v the two components of the stream velocity in 


y 



Figure 19. Details of tracings of photographed 
wave fronts. 

the direction of the two coordinate axes, c the 
local sonic velocity, and t the time. 

Let us assume that many photographs of 
sound waves generated by a multiple spark 
gap (such as shown in Figure 8) are available, 
taken at successive known time intervals. If 
all pictures are superimposed and traced, a 


grid such as shown in Figure 18 will result. 
Define the coordinate axes and draw a line 
through the point of interest P(x 0 ,y 0 ) parallel 
to the y axes. (See Figure 19.) Along this line 
the distances above the x axes, at which suc¬ 
cessive wave fronts originating at the same 
source intersect the line, can be measured. 
Since the time increments are known, a curve 
y vs t for x 0 = constant can be drawn and 
hence dy/dt at x 0 determined. The slopes, 
dy/dx, of the wave fronts where they intersect 
the line x = x 0 can also be measured, and by 
interpolation, dy/dx at P can be calculated. 

This procedure is repeated for two other 
families of wave fronts emanating from two 
other spark gaps. Substituting the three pairs 
of values of dy/dx and dy/dt into equation (1) 
three simultaneous equations, linear in u, v, 
and c, are obtained which can be solved easily 
for the three unknowns at P(x 0 ,yo)- 

In the second method based on the wave 
front equation (1) only two families of waves 
are evaluated as described above. As the third 
equation the Bernoulli equation is used. A 
knowledge of the temperature of the undis¬ 
turbed air before entering the tunnel is re¬ 
quired. 

The estimated probable error in the deter¬ 
mination of the principal stream velocity is 
about 15 per cent. It is believed that this figure 
can be improved upon by improvement of the 
experimental procedures involved. The merit 
of both methods lies in the fact that the spark 
gaps can be mounted flush with the tunnel 
bottom and hence the disturbance to the stream 
can be made negligible. 












Chapter 18 

AUDITORY SIGNALS FOR INSTRUMENT FLYING: "FLYBAR” 


T he large number of operations required 
of the pilot of a modern airplane and the 
multitudinous instruments which he must fol¬ 
low tax his abilities to the limit. His eyes espe¬ 
cially are taxed, since he must keep track of 
the large number of instruments on the panel 
and at the same time see obstacles, other air¬ 
planes, and the ground at critical times. In the 
larger airplanes both pilot and co-pilot are 
sometimes kept busy, especially when flying 
under instrument conditions. 

The continuously increasing speed of modern 
planes and the advent of new devices such as 
airborne radar will, if anything, increase this 
critical load on the eyes. Thus it would be an 
advantage if some of the flight indications 
could be furnished to the pilot by way of his 
ears. 

As early as 1936 it was demonstrated 1 that a 
pilot could fly an airplane with his eyes blind¬ 
folded when two instrument indications were 
given by means of an auditory signal in his 
earphones. The pilot flew with rudder and ele¬ 
vators only, allowing the inherent stability of 
the Fairchild 24 to take care of lateral control. 
The signals employed were (1) a turn indica¬ 
tion consisting of increase of the signal in¬ 
tensity to one ear and decrease of intensity to 
the other, and (2) an airspeed indication con¬ 
sisting of a change of pitch of the signal. Al¬ 
though the plane described a wide climbing 
spiral rather than a straight path, the pilot 
was able to maintain satisfactory control and 
to recover from spins, thus showing the feasi¬ 
bility of flying by auditory reference, flybar. 

The purposes of the present study 2 were to 
determine (1) what types of auditory signals 
could be followed with greatest ease, (2) with 
what accuracy such signals could be followed, 
and (3) how many simultaneous auditory sig¬ 
nals could be followed successfully. It was, 
therefore, necessary to design a variety of 
different auditory signals and to test their 
effectiveness on a group of men who were 
performing a task similar to that of “flying 
blind” in an airplane. 


The results indicate that if the signals are 
properly designed as many as four auditory 
indications can be followed without interfering 
with radio and interphone communication when 
occasion demands. It is thought that such sig¬ 
nals may be of value not only in military avi¬ 
ation but also in peacetime flying. 


181 DEVELOPMENT OF AUDITORY 
SIGNALS 

1811 Tone Signals 

The aim of this part of the investigation was 
to develop three aural indications which could 
be followed simultaneously. Indications for 
turn, bank, and airspeed were tried out on two 
synthetic devices. One of these was the airplane 
pursuit meter, a device in which the subject 
tried with controls similar to those in an air¬ 
plane to compensate for deviations introduced 
in an unpredictable fashion by a mechanical 
device. Automatically computed scores indicated 
the success with which the signals were fol¬ 
lowed. The second device used was the familiar 
Link Trainer, fitted with auditory signals to 
give turn, bank, and airspeed indications. The 
records obtained by use of the auditory signals 
were compared with records made by the same 
individuals using visual indications. 

Ten men between the ages of nineteen and 
thirty-six were used as subjects. Since none 
of these men had had previous flying training, 
the course of learning could be followed for 
both the visual and auditory types of indication. 
The task, in the case of the Link Trainer, was 
to fly a straight course by means of turn, bank, 
and airspeed indications alone (without any 
compass or gyro direction indicator) and with 
the rough-air attachment turned on. The task 
set by means of the airplane pursuit meter was 
a somewhat similar one. Several trained pilots 
also tried the best signals on the Link Trainer. 

A number of different types of auditory sig¬ 
nals were experimented with, the first being 


255 


256 


AUDITORY SIGNALS FOR INSTRUMENT FLYING: “FLYBAR 


combinations of buzzes, pitches, “chopped” sig¬ 
nals, and other conventional tone signals. In 
general, it was found that combinations formed 


when two or more signals were used. There was 
a general tendency when using these complex 
signal combinations for the subject to follow 


SIGNAL GENERATOR 


MODULATOR AND MIXER 



Figure 1. Relay connections and block diagram for the successful three-in-one auditory indication. 



Figure 2. The Link Trainer with the signal 
equipment mounted for test purposes. 

by adding separate tone signals together were 
too complex to be followed successfully. Some 
individuals had difficulty in analyzing each in¬ 
dividual signal in the complete auditory pattern 


one indication with such attention that the 
other two escaped him and “went out of con¬ 
trol.” 

Two auditory signal combinations were de¬ 
veloped, however, which were successful enough 
to be of possible practical utility. Both “sounded 
like the behavior of the airplane.” One involved 
apparent motion of the tone to the right or to the 
left in a manner roughly corresponding to the 
right and left indications of a radio compass. 
This signal can be used with a second indica¬ 
tion superimposed on it to indicate airspeed. 

The second successful signal was a three-in- 
one indication. That is to say, different charac¬ 
teristics of the same signal were used to indi¬ 
cate turn, bank, and airspeed. These indications 
were, respectively, (1) a repetitive or sweeping 
type of motion of the signal from left to right, 
or right to left, (2) an apparent “tilt” pro¬ 
duced by pitch variations, and (3) a “putt” 
sound, the rate of occurrence of which could 
be associated with the sound of the airplane 
motor. A block diagram of the device used to 
generate this signal is shown in Figure 1. 




















































































































































DEVELOPMENT OF AUDITORY SIGNALS 


257 


With this three-in-one signal the ten subjects 
learned to operate the Link Trainer so as to 
fly a respectably straight course in “rough air” 
after from 2 to 21/2 hours of practice. The re¬ 
sults corresponded quite well with those ob¬ 
tained by the same men using visual indications 
after IV 2 to 2 hours of practice. Figure 2 shows 
the signal equipment mounted on the Link 
Trainer for test purposes while Figures 3 and 4 
give sample records for one subject using both 



Figure 3. Link Trainer records for subject 
using visual indications. 


types of indication. As would be expected, some 
of the men had more difficulty than others, both 
in learning to manipulate the controls and in 
following each type of signal. 

Six private pilots, some of whom had had 
instrument instruction, were able to operate the 
Link Trainer creditably by means of the audi¬ 
tory signals in from 1 to li/o hours of practice. 
Several Navy pilots, after a short trial, were 
confident that it would be possible to fly with 
this signal combination. 

The two-in-one signal did not give quite so 
accurate results as the three-in-one but seemed 
good enough to be of possible use. Either of the 


two signal combinations might be applied to 
other instruments than the ones used for test 
purposes. For instance, the turn signal might 
be of more use if controlled by the gyro direc¬ 
tion indicator or the radio compass for keeping 
on course on long flights. 

It was of especial interest that the simulated 
radio-range signal and voice communication 
could be heard simultaneously with the two-in- 
one and the three-in-one type of signal without 



Figure 4. Link Trainer records for subject 
using auditory indications. 


difficulty. This was due to avoidance of a large 
number of tones which would cause greater 
interference with range and voice signals. 


18-1,2 Automatically Produced Speech 
Signals 

A device called an automatic annunciator was 
developed for the purpose of announcing alti¬ 
tude, airspeed, or other similar instrument in¬ 
dications directly to the pilot. This device trans¬ 
lated the indicator readings automatically into 
spoken messages. It proved quite successful and 





















258 


AUDITORY SIGNALS FOR INSTRUMENT FLYING: “FLYBAR” 


offered promise of useful application for many 
types of instrument indications and of warn¬ 
ings. 

As developed for demonstration purposes, 
the device announced altitude in 200-ft units 
through the pilot’s earphones. The annunciator 
consisted of a light, compact, multichannel 
sound reproducer of the magnetic tape type, 
on each channel of which a permanent message 


in order to say, for example, “two-thousand 
two-hundred feet.” 

The automatic annunciator may be coupled 
to instruments by servo devices in cases where 
the aircraft instrument does not have sufficient 
torque to operate the control device directly. 
For some applications, however, the annunci¬ 
ator may be operated directly (without servo 
unit) by means of vacuum tubes. 



Figure 5. Basic assembly of multichannel sound reproducer. 


had been previously recorded. Each channel 
contained one spoken number or one unit of 
the message and these were selected by relays 
operated by a control-switch unit. This unit 
followed the Link Trainer altimeter by means 
of a self-synchronous repeater and servo units. 
After the appropriate message units were se¬ 
lected they were played in the proper sequence 


The control circuit is such that when using 
a four-unit message such as “four-thousand 
four-hundred feet” the lag in the spoken indi¬ 
cation will never be more than the length of 
the message plus 1 sec. In the four-unit message, 
this means a lag of 5 sec. When the application 
requires the shortest possible time interval, 
single-channel messages may be used to reduce 







POSSIBLE APPLICATIONS 


259 


the lag to about 1 sec, or if 1-sec lag is still too 
long a circuit of the anticipation type can be 
employed. 

The twenty-four channel magnetic repro¬ 
ducer developed for annunciator use a weighed 
approximately 15 lb. Associated selection and 
control equipment will weigh more or less de¬ 
pending on the application. Figure 5 shows 
the multichannel sound reproducer and Figure 
6 illustrates it covered and connected to a relay 



Figure 6. Complete annunciator—relay and se¬ 
quence unit at the left, reproducer at the right. 


and sequence unit developed for use with the 
altimeter. 

It is of interest that tests with the annunci¬ 
ator on the Link Trainer altimeter have shown 
that there is very little difficulty in distinguish¬ 
ing between speech from the annunciator and 
incoming speech communication from outside 
(radio) channels. Communication messages may 
be made to override the annunciator, and, also, 
the annunciator has a repetitive rhythm and a 
speech quality which is readily identified. 


18.2 FUNDAMENTAL principles of 
SUCCESSFUL AURAL INDICATIONS 

In order to be successful, auditory signals 
must be designed with a view to certain psy- 

a Through the cooperation of Bell Telephone Labora¬ 
tories. Appreciation is expressed to Dr. E. C. Wente of 
NDRC Section 17.3 and Mr. W. L. Woolf of Stevens 
Institute of Technology for valuable advice and interest. 


chological principles related to hearing and to 
the pilot’s reactions. 

1. Pilots have certain habitual methods of 
thinking about the airplane. Auditory signals 
must be designed to fit these habitual methods 
of thinking. 

2. Most fliers are much more accustomed to 
using visual indications than auditory ones. 
The signals should, therefore, be as simple and 
as self-explanatory as possible. Only under 
these conditions will the need for special train¬ 
ing of pilots in use of the signals be reduced to 
a minimum. 

3. It can be demonstrated that a person can 
attend independently to only one thing at a 
time. It was found that when several different 
auditory signals were added together the indi¬ 
vidual following the signals might attend for 
a while to each in turn, as he was supposed to 
do. Then, however, there was a tendency for 
one signal to “capture” his attention to the 
exclusion of the others. To be of practical value, 
the auditory signals must be designed so that 
this “capture of attention” does not occur. 

4. In military aircraft, information from var¬ 
ious sources is continuously obtained through 
the earphones from radio and interphone chan¬ 
nels. Any auditory indications used must be 
designed to interfere as little as possible with 
such messages. 

5. It is not sufficient to produce an auditory 
signal which satisfies the designer. The signals 
must also be tested by psychological methods 
on persons unfamiliar with their development. 
In so doing, preconceived prejudices and special 
abilities found among particular men will be 
eliminated. If this is not done, erroneous con¬ 
clusions may result. 


183 POSSIBLE APPLICATIONS 

It is not anticipated that auditory signals 
will entirely displace visual instruments in the 
airplane for any of the fundamental indications. 
However, it has been demonstrated that as 
many as three auditory signals can be followed 
with accuracy if properly designed aural indi¬ 
cations are used. It is thought that the spoken 
indications can be useful as auxiliary aids to 




260 


AUDITORY SIGNALS FOR INSTRUMENT FLYING: “FLYBAR 


furnish warnings of various types and to give 
altitude and airspeed information to the pilot 
whenever his eyes must be otherwise engaged. 

A tone signal attached to the gyro direction 
indicator might be of considerable advantage 
on airplanes which do not have the automatic 
pilot for maintaining the proper heading on 
cross-country flights. Or again, such a signal 
might be controlled by the self-orienting loop 
of a radio compass for long distance “homing” 
on radio stations. This would allow the pilot 
to maintain the proper bearing on the station 
without the necessity of constantly watching 
the needle indicator of the radio compass. 

It is also possible that auditory signals, if 
properly designed, can be of assistance in con¬ 
nection with some of the new blind-landing 
systems which are under development. 


The automatic annunciator can be arranged 
to announce altitude or airspeed or both alter¬ 
nately for landings and take-offs. Such auditory 
messages would be of advantage in single-seater 
planes for landings made under conditions of 
low visibility. In larger airplanes the annunci¬ 
ator could also relieve the co-pilot of calling 
this information. Furthermore, the device can 
be arranged to call attention to sources of 
trouble such as “wheels not down,” “flaps not 
down,” “gas tank empty,” or other such items. 

In practice, of course, the slight added weight 
of equipment must be weighed against advan¬ 
tages gained. Auditory signals have a distinct 
advantage, however, wherever the pilot’s at¬ 
tention must be called to a malfunctioning part 
or whenever his eyes must be otherwise occu¬ 
pied. 



Chapter 19 

SPECIAL DEVICES FOR USE ON SHIPBOARD 


I N this chapter a brief discussion is given 
of certain special equipment developed at 
the Electro-Acoustic Laboratory for use aboard 
ship. The equipment was designed primarily to 
facilitate and simplify the flow of information 
of various kinds about the ship during battle. 


191 THE RADIO REPEAT UNIT 

There are indications that present voice com¬ 
munication channels on shipboard may become 
overloaded at critical times. This is especially 
true of the radio channels, since they share the 
largest part of the burden of external com¬ 
munications. When messages must be repeated 
on the air, either because of poor intelligibility 
or simply because the receiving operator is not 
available on the particular circuit at the time, 
congestion results and a breakdown may occur 
during times of high-circuit load. 

One of the simplest ways of increasing the 
working efficiency of a given communication 
channel is to eliminate all on-the-air repeats. 
A request was made in 1944 to the Electro- 
Acoustic Laboratory by the U. S. Navy Bureau 
of Ships to develop a simple device which would 
be capable of quickly repeating any radio mes¬ 
sage without interfering with the normal oper¬ 
ation of the radio channel over which the 
message was received. 

The radio repeat unit 3 [RRU] was subse¬ 
quently developed at the Electro-Acoustic Lab¬ 
oratory, and a model was successfully tested. 

The basic elements of the RRU are a record¬ 
ing unit consisting of an endless magnetic tape, 
a recording head, erasing head, and a playback 
head with suitable switching facilities. Figure 1 
shows a simplified diagram of the recording 
unit. When switch aS^ is closed, any incoming 
signal will be recorded magnetically on the 
tape. Any message which might have been on 
it earlier is erased by the erasing head at posi¬ 
tion 3. The recorded signal, delayed by a certain 
time interval, is carried by the tape around 
the loop and played back into the earphones 


(S 2 closed) by the playback head at position 2. 
This interval of delay is determined by the 
length and speed of the tape. If switch S 1 is 
opened, all further recording and erasing ceases 
and the message recorded on the tape is stored 
for an indefinite number of future playbacks. 

The tape of the required length is reeled on 
a magazine shown in the center of Figure 2. 
In operation, the tape emerges from the maga¬ 
zine near the center, then passes the playback, 
erasing and recording heads, and goes around 
the driving drum to be pushed back into the 



Figure 1 . Simplified diagram illustrating the 
principle of the radio repeat unit. 


magazine from the outside through a guide. 
An alarm indicator is provided to indicate tape 
slack or breakage. 

The magazine and tape drive are of novel 
design, and both have performed well in life 
tests in the laboratory. The magazines can be 
easily replaced in case of defect or if a different 
time delay is desired. 

Conventional, longitudinal, magnetic-record¬ 
ing technique is used with an a-c bias current 
in the recording head. A tape speed of about 
3 ft per sec yields a satisfactory frequency re¬ 
sponse up to about 4,000 c. The tape is made of a 
stainless-steel alloy. A signal-to-noise ratio of 
at least 40 db is easily achieved. 311 

The complete RRU has three recording loops. 
This makes it possible to incorporate three dif¬ 
ferent delay times or storage of a message on 
one tape while others are being recorded on the 
remaining two loops. A simplified diagram of 
the RRU, incorporating delay times of 15, 30, 


261 











262 


SPECIAL DEVICES FOR USE ON SHIPBOARD 


and 60 sec, is shown in Figure 3. In Figure 4 
the three tape units are shown mounted side 
by side. A sketch of the complete unit with 


Some of the useful functions of the RRU can 
be listed as follows. 

1. Quick repeat of part or all of a message 



r—ALARM 
INDICATOR 


PLAYBACK HEAD , -MAGAZINE CONTAINING TAPE 


ERASING HEAD 


-RECORDING HEAD 


DRIVING DRUM 


GUIDE 


Figure 2. Tape unit. 



Figure 3. Diagram of radio repeat unit with 
three recording loops. 


three remote stations is shown in Figure 5. The 
details of the switching arrangements can be 
easily varied to suit the needs of various sta¬ 
tions and ships. 



Figure 4. Tape units of radio repeat unit. 


so that requests for repetition are eliminated 
and the circuit load is decreased correspond¬ 
ingly. 

2. Two simultaneously arriving messages on 














































THE RADIO REPEAT UNIT 


263 



REMOTE STATION 


REMOTE STATION REMOTE STATION 

- 2 - - 3 - 

FiGURE 5. Complete radio repeat unit with three remote stations. 





























































































































264 


SPECIAL DEVICES FOR USE ON SHIPBOARD 



FREQUENCY 
POWER SUPPLY 

Figure 6. Schematic representation of the method of indicating time on voice recordings. 



Figure 7. Timing pulse generator, recorder-reproducer, and timing pulse receiver (laboratory models). 















































A METHOD OF TIME INDICATION ON VOICE RECORDINGS 


265 


two different channels for the same recipient 
can be separated in time by inserting a suitable 
time delay. 

3. Temporary or permanent storage of im¬ 
portant messages. 



Figure 8. Recorders installed on shipboard. 


4. Assistance to monitoring personnel in 
keeping complete logs, especially if more than 
one channel has to be supervised. 


tioned above 3b along with a detailed description 
of the device. 


192 A METHOD OF TIME INDICATION 
ON VOICE RECORDINGS 

Recordings of the intelligence transmitted by 
the important voice communication systems on 
shipboard are potentially useful in many cases. 
However, such recordings can be fully utilized 
only if an accurate indication of the time is 
also provided for on the record. By installing 
multichannel voice recording equipment on 
shipboard, continuous voice and time recordings 
can be made during action. A complete and 
permanent battle record is thus obtained which 
should prove useful in a critical evaluation of 
the performance of equipment and personnel. 

Equipment capable of superimposing suitable 
time indications on voice recording without det¬ 
rimental interaction and without using addi¬ 
tional channels was designed and constructed 
at the Electro-Acoustic Laboratory. 1 

A schematic representation of the method of 



Figure 9. Photograph of coordinate system projected on plotting surface on DRT. 


The usefulness of an RRU can be extended indicating time is shown in Figure 6. The power 
beyond the obvious cases given above, and such line with a controlled frequency of 60 c is used 
applications are discussed in the report men- as the standard of time. Controlled by this 









266 


SPECIAL DEVICES FOR USE ON SHIPBOARD 


power line, the timing pulse generator [TPG] 
generates a suitably codec! time signal which 
is superimposed on the voice signal and simul¬ 
taneously recorded on any number of recorders. 
Interaction between the two signals is pre- 



Figure 10. Diagram of point-source projector. 


vented by suitable frequency-discriminating 
networks. Upon playback, the time and voice 
signals are separated again by filters. The tim¬ 
ing pulse receiver [TPR] converts the coded 
information of the timing pulses into a visual 
indication of the time at which that portion of 



Figure 11. Laboratory model of point-source 
projector mounted on Arma class 2 DRT. 


the voice recording was made (2012, or 8:12 
p.m., in Figure 6). 

The time signal is transmitted from the TPG, 
not continuously but at regular intervals. The 
recorded indications thus form a set of “fixes,” 
or known points on the record. Errors caused 
by variations in the speed of the medium are 


automatically rectified when the next “fix” 
occurs. Therefore, the errors are not cumu¬ 
lative, and long periods of recording and dis¬ 
continuities of time in the recorded material do 
not affect the accuracy of the indications. 

As the number of indications per unit time is 
increased, approaching in the limit a contin¬ 
uous indication, the errors accumulated between 



MOLYBDENUM ANODE 


TANTALUM CYLINDER 


TUBE FILLED WITH ARGON 


HOLLOW CENTER OF TANTALUM 
CYLINDER RACKED WITH 
ZIRCONIUM OXIDE 


Figure 12. Construction of point-source tube. 


“fixes” become negligible. One-minute intervals 
were chosen as a practical spacing. 

Exactly once every minute, then, the time is 
identified on a 24-hour basis by suitably coded 
pulses generated in the TPG. The 1,440 minutes 
are identified by four groups of pulses. 



Figure 13. Photograph showing point-source 
tube. 

In order to record a series of pulses with 
conventional equipment, a suitable “carrier” 
frequency was chosen which is modulated by 
the series of pulses comprising the time signal. 
By use of suitable high-pass and low-pass filter, 
the carrier frequency is accommodated slightly 























A METHOD OF TIME INDICATION ON VOICE RECORDINGS 


267 


above the upper cutoff frequency of the voice 
channel. 

Laboratory models of the TPG and TPR are 


application at hand. Continuous recordings of 
over 1 hour duration were made in the labora¬ 
tory and on shipboard. Figure 8 shows a bat- 




Figure 14. Photograph of horizontal plotting table showing own ship’s course projection. 


shown in Figure 7. A commercial recorder- tery of recorders installed on shipboard for an 
reproducer, using an endless plastic tape as analysis of the circuit loads carried by the vari- 
recording medium, was modified to suit the ous voice communication channels. 












268 


SPECIAL DEVICES FOR USE ON SHIPBOARD 


Details of the operational and constructional 
features of the TPG and TPR have been com¬ 
piled in report form. la The laboratory models 
of the TPG and TPR were tested and inspected 
at the Naval Research Laboratory, and many 
of the important features were incorporated in 
a new design proposed by the Navy. 


In cooperation with Section 634, U. S. Navy 
Bureau of Ships, a simple lightweight projector 
was developed at the Electro-Acoustic Labora¬ 
tory. 23 The device is designed to be mounted on 
the pencil carriage of the DRT, and projects a 
a polar coordinate system 26 in. in diameter 
onto the plotting surface (see Figure 9). Di¬ 
rect plotting of radar contacts is thereby greatly 



Figure 15. Edge-lighted Lucite display board 
(5x5 ft). The lower photograph was taken in 
the dark. 


193 A POINT-SOURCE PROJECTOR FOR 
USE ON DEAD-RECKONING TRACERS 

Facilities for rapid and accurate plotting of 
radar data on the dead-reckoning tracer [DRT] 
are very important. Universal drafting arms 
and other auxiliary plotting devices have not 
proved satisfactory. 




Figure 16. Edge-lighted Lucite status board 
(3x3 ft). The lower photograph was taken in the 
dark. 

facilitated. If a plotting scale of 2,000 yd per in. 
is used, a circle of 13 miles radius about the 
ship is covered. 




















LIGHTING OF PLOTTING AND DISPLAY SURFACES 


269 


To project the image of the coordinate system 
on the plotting surface a point-source projector 
is used. It consists of a lightweight case, a glass 
plate with the coordinate system to be pro¬ 
jected, and a tube which supplies the “point” of 
light of required intensity. Figure 10 shows, 
diagrammatically, a laboratory model of the 



Figure 17. Horizontal plotting table with in¬ 
direct lighting. 


projector mounted on the carriage of a stand¬ 
ard DRT (Arma). A photograph of the device 
is shown in Figure 11. 

The light source is a gas-filled tube with two 
electrodes. After a low-impedance ion path 
between cathode and anode has been started 
by a surge from the power-supply unit, an arc 
is maintained between the two electrodes with 
a voltage drop of about 20 v. The cathode con¬ 
sists of a cylinder of tantalum whose center is 
filled with zirconium oxide. The area of this 
core showing on the end face of the tantalum 
cylinder becomes incandescent under ion bom¬ 
bardment. The anode surrounds the cathode, 
and by a suitable opening a usable projection 
angle of 120 degrees is obtained. Figures 12 and 
13 show the point-source tube in detail. 


In its present form the power consumption 
of the tube is about i /2 w, and its average life¬ 
time is about 500 hours. A suitable power- 
supply unit with arrangement for starting the 
arc is discussed in the report mentioned above. 2b 
Other applications for a light source of this 
type suggest themselves. As an example, Figure 
14 shows a photograph of the image of a true 
relative bearing conversion scale projected on a 
plotting table of conventional design by means 
of the light source. 

19 4 LIGHTING OF PLOTTING AND 
DISPLAY SURFACES 

The lighting problem on shipboard is an im¬ 
portant one. Proper lighting of the plotting and 
display surfaces goes a long way toward efficient 
operation of information systems as a whole. A 
high level of light intensity is not permissible, 
owing to the presence of cathode-ray tubes used 
as remote radar indicators. It is, therefore, 
necessary that the plotting and display boards 
be designed for use in low ambient light in¬ 
tensity. 

The device of using a suitable plastic as 
plotting surface and introducing light through 
the edges by means of fluorescent tubes has 
been shown to yield good results for vertical 
plots. 2 Figures 15 and 16 show vertical plots of 
the display and status board types photographed 
in light and in the dark. The lines traced with 
grease pencil are clearly visible. 

In addition to vertical plotting surfaces, con¬ 
sideration must be given to the horizontal plot¬ 
ting tables. It is essential that the illumina¬ 
tion of the plotting surface be diffuse for mini¬ 
mum eye strain. A method of indirect lighting 
has been devised at the Electro-Acoustic Lab¬ 
oratory. Fluorescent lamps are spaced around 
the table in such a way that their light is 
introduced into the inside of the metal cone 
below the plotting surface by suitable openings. 
No direct light can thus reach the plotting 
surface. Flat white paint is used to coat the 
inside of the cone to achieve diffusion. Mechan¬ 
ical shutters control the light intensity. 

Figure 17 shows a schematic section through 
a plotting table using indirect lighting of the 
type described. 






































































GLOSSARY 


a. Absorption coefficient. 

Accommodation, Speed of. In vision, the speed with 
which the shape of the eye lens changes as the in¬ 
dividual refocuses for a different distance. 

AD. Altitude decrement, the ratio of the response of a 
device at sea level to its response at altitude. The 
response may be expressed in terms of the sound 
pressure of an earphone, the voltage of a microphone, 
or the gain of an amplifier, or of a whole communica¬ 
tion system. In that case, it may include the effects 
of altitude on listener and talker. 

AI. Articulation index, an empirical function with 
additive properties, directly related to the articulation 
score obtained with a given crew of talkers and 
listeners, using specified speech material. 

Airspeed, True. Speed of aircraft relative to the air 
at a large distance. 

Amplitude Distortion. A device is said to introduce 
amplitude or nonlinear distortion if the output is 
not linearly related to the input. 

Anechoic Chamber. Room free from acoustic wall re¬ 
flections (echoes). 

Articulation. The articulation test is a procedure by 
which a quantitative measure of the intelligibility of 
speech can be obtained. The articulation score is the 
percentage of the items received correctly by the 
listeners for the conditions under test. Syllable, word, 
or sentence articulation scores are obtained with 
meaningless syllables, isolated words, or complete 
sentences, respectively, as the test material. 

Artificial Ear. Cavity with calibrated test microphone 
simulating the human ear for testing earphones. 

Artificial Voice. Sound source simulating a human 
talker for testing microphones. 

ASA. American Standards Association. 

ATB/ARB. Naval aircraft transmitter and receiver. 

Attenuation. The number of decibels by which the 
sound intensity is reduced by the structure in ques¬ 
tion. This may be a panel, wall earphone socket, or 
noise shield. 

Audiogram. Auditory threshold as measured by a 
standard audiometer, plotted as a function of fre¬ 
quency. 

Automatic Annunciator. A device which announces 
altitude, airspeed, or similar instrument indications 
directly to the pilot’s earphones. 

B. Spectrum level of noise at the listener’s ear. 

Spectrum level of speech at the listener’s ear. 

bhp. Brake horsepower—the power delivered by the 
engine to the propeller, in hp. In multi-engine air¬ 
craft the power per engine is always given. 

Binaural. Stimulation presented to both ears. 

Boundary Layer. Region of the airstream near the 
surfaces bounding the flow. 

BTL. Bell Telephone Laboratories. 

Carrier Sentence. In articulation testing, a phrase, 
such as “you will write-,” which the talker uses to 


monitor his voice level and which the listeners use as 
a signal for listening. 

Center Clipping. A type of amplitude distortion. A 
center-clipping or peak-pass circuit will pass only 
peaks exceeding some fixed value. 

Circumaural Socket. Earphone socket designed to seal 
against the head around the ear with a cavity large 
enough to accommodate the pinna. 

Coupler. (1) Artificial ear. (2) Earphone socket or tip. 

Critical Bandwidth. The width of the band of noise 
(of the continuous type of spectrum) necessary for 
the total energy in the band to be equal to that of a 
given pure tone at its masked threshold. The critical 
bandwidth is conveniently expressed in decibels, K. 

DRT. Dead Reckoning Tracer. 

/ 7? T 7 x 

E = 4 / -. Efficiency index of a soundproofing struc- 

V dm 

ture. 

Earphone Socket. Socket or cushion which couples 
acoustically the earphone unit to the ear. 

Ear Warden. An aural protective device designed to 
exclude intense and undesirable noises from the ear. 
Ear plug. 

Equal-Articulation Contour. Contour relating the 
bandwidth of a voice communication system to the 
level of speech, with articulation score as parameter. 

Equivalent Free-Field Sound Pressure. The free-field 
sound pressure of a signal which sounds as loud (to 
a normal listener seated in a quiet room free from 
acoustic wall reflections) as a signal of the same type 
and frequency introduced over earphones. The free- 
field sound pressure is measured at the listener’s 
location, and is generated by a sound source located 
some distance in front of him. 

/. Frequency, in cycles per second (c). 

Flybar. Instrument flying by auditory, rather than 
visual, signals. 

Frequency Distortion. A device is said to introduce 
frequency distortion if the efficiency with which 
different frequencies are transduced is not constant. 

Gain Function. Function relating the articulation 
scores to the gain of the system under test, for quiet 
or fixed conditions of noise. 

Handset. A combination of a single earphone and 
microphone joined by a suitable handle. 

HCCR. Central Communications Research, Cruft Labo¬ 
ratory, Harvard University. 

Headset. A combination of two earphones and earphone 
sockets (cushions or tips) and a suitable headband 
and cord. 

HEAL. Electro-Acoustic Laboratory, Cruft Laboratory, 
Harvard University. 

Hearing Loss. Shift in the auditory threshold from the 
normal, or reference value. 

HPAL. Psycho-Acoustic Laboratory, Harvard Uni¬ 
versity. 

IAS. Indicated airspeed—the reading of the airspeed 
indicator in mph. 


271 




272 


GLOSSARY 


Impervious Septum. Layer of material with infinite flow 
resistance. 

Ind. Alt. Indicated altitude—the reading of the altim¬ 
eter without correction, in feet. 

Insert Tip. Earphone socket designed to be inserted 
into the auditory canal. 

Interphone. A voice communication system consisting 
of microphone, earphones, and intermediate trans¬ 
mitting network. 

JASA. Journal of the Acoustical Society of America. 

•Jamming. Countermeasures used to deprive the enemy 
of information normally received via his communica¬ 
tion systems. 

K. Width of a critical band of frequencies, in decibels. 

Listening Test. Test of listening ability, usually ad¬ 
ministered to a group of listeners. 

Loudness. That aspect of auditory sensation in terms 
of which sounds may be ordered on a scale running 
from “soft” to “loud.” Loudness is chiefly a function 
of the intensity of a sound, but is also dependent upon 
the frequency and the composition. The unit is the 
sone. 

Loudness Level. The loudness level, in phons, of a 
sound is numerically equal to the intensity level in 
decibels of a pure tone of 1,000 c which is judged by 
the listeners to be equal in loudness. 

M. Masking. 

Mach Number. Ratio of stream velocity to sonic 
velocity at a point in the airstream. 

MAF. Minimum audible field. Free-field sound pressure 
level at threshold in quiet at the observer’s location. 

MAP. Minimum audible pressure. Threshold sound pres¬ 
sure level in quiet, measured at the observer’s ear¬ 
drum. 

MF — —. Merit factor of a soundproofing structure. 

Masking. Masking is defined as the number of decibels 
by which a listener’s threshold of audibility is raised 
in the presence of another sound. 

Maximum Cruising Power. Power condition of an air¬ 
plane engine corresponding to about 60 to 70 per 
cent of rated power. 

Mel. The unit of pitch. It is so defined that a 1,000- 
cycle tone 40 decibels above threshold has a pitch of 
1,000 mels. (The mel is a so-called “subjective” unit.) 

Military Power. Power condition of an airplane engine 
corresponding to the maximum power available. 
About 110 to 125 per cent of rated power. It cannot 
be maintained for more than several minutes. 

Monaural. Stimulation presented to one of the two 
ears. 

« ;j . Difference between actual average attenuation N ?i 
and average attenuation predicted by weight law, in 
decibels, at 3,000 cycles. 

n-. Difference between actual average attenuation N- 
and average attenuation predicted by weight law, in 
decibels, at 5,000 cycles. 

N = 10 log 1/r. Attenuation of an acoustic structure, 
in decibels. 


N-N. Noise-to-noise: Both announcer and listeners are 
located in an ambient noise field. 

N-Q. Noise-to-quiet: The announcer is located in an 
ambient noise field, while the listeners are in the 
quiet. 

NACA. National Advisory Committee for Aeronautics. 

NDRC. National Defense Research Committee. 

Near Point of Vision. The shortest distance at which 
a subject can accurately focus his two eyes on an 
object. 

Noise-Canceling Microphone. A microphone respond¬ 
ing primarily to the pressure gradient in a sound 
wave. It is designed for use close to the talker’s lips 
(“lip” microphone). 

Noise-Peak Limiters. Peak-clipping devices designed 
to eliminate static peaks from radio receivers. 

Noise Shield. An enclosure worn over the talker’s lips 
and sealing well to his face to shield the microphone 
contained in it from unwanted external sounds. 

Normal Cruising. Power condition of an airplane 
engine well below maximum cruising power. 

oi — 2nf. Angular frequency in radians per second. 

Orthotelephonic Gain [OG]. The number of decibels 
by which the signal at the listener’s ear, when the 
communication system in question is being used, 
differs from the signal which would exist at his ear 
if the talker, talking in the same way, were com¬ 
municating with him over a free path of air one 
meter in length in a quiet room free from acoustic 
wall reflections. 

Orthotelephonic Reference System. A communica¬ 
tion system, consisting of a normal listener and 
talker who face each other at a distance of one meter 
in a quiet room free from acoustic wall reflections. 

OSRD. Office of Scientific Research and Development. 

Paper Form Board. (Revised Minnesota Form Board 
Test.) The testee is required to manipulate geometric 
shapes to fill a prescribed area. The score is the 
number of correct answers given in a limited period 
of time. 

Peak Clipping. A type of amplitude distortion. A peak¬ 
clipping or peak-limiting circuit does not pass peaks 
exceeding some fixed value. 

Peak Factor. Ratio of peak to rms value in a complex 
wave. 

Phon. A unit for measuring the loudness level of a tone. 

Phoneme. Minimum distinctive unit of speech sound. 

Phonetic Alphabet. A set of symbols representing 
speech sounds. In military usage, the term refers to a 
list of words used in speech communication to identify 
the individual letters of the alphabet (Able, Raker, 
Charlie, etc.) 

Pitch. That aspect of auditory sensation in terms of 
which sounds may be ordered on a scale running from 
“low” to “high.” Pitch is chiefly a function of the 
frequency of a sound, but is also dependent upon the 
intensity and the composition. The unit is the mel. 

Premodulation Clipping. Clipping the peaks of the 
speech signal prior to modulation of the amplitude of 
an r-f carrier wave. 



GLOSSARY 


273 


Pressure Altitude. Altitude in feet, defined by the 
U. S. Standard Atmosphere, at which the atmospheric 
pressure is equal to the given pressure. 

PRF. Pulse repetition frequency. 

Propeller-Tip Passage Frequency. Frequency at which 
the propeller tips pass a fixed plane through the 
propeller axis, in cycles per second. 

Psychomotor Efficiency. The efficiency, measured by 
speed or by the number of errors, of the muscular 
responses to cerebral processes. 

Pursuit Meter. An instrument for measuring eye-hand 
coordination, which combines (1) a stimulus target, 
the movements of which the subject endeavors to 
follow, and (2) an integrating meter which registers 
the amount of coordination error in terms of ampli¬ 
tude or time or both. 

Q-N. Quiet-to-noise: the announcer is located in the 
quiet, while the listeners are in an ambient noise field. 

Q-Q. Quiet-to-quiet: both announcer and listeners are 
located in the quiet. 

R. Flow resistance. 

R = lim p/v. Flow resistance, in grams per square 
centimeter per second. 

Rated Power. Power condition of an airplane engine 
corresponding to maximum continuous power avail¬ 
able. Also called maximum (continuous) power. 

Real-Ear Response. The real-ear response of an ear¬ 
phone is the ratio of the equivalent free-field sound 
pressure to the voltage applied across the terminals. 
The determination of the equivalent free-field pres¬ 
sure requires a judgment of equal loudness for tones 
of various frequencies. 

Real-Voice Response. The real-voice response of a 
microphone is the ratio of the microphone output 
voltage to the free-field sound pressure measured one 
meter in front of the lips of a normal talker wearing 
the microphone in the normal manner. Usually the 
talker uses a representative speech sample and the 
real-voice response is determined in contiguous bands 
of frequencies by suitable analyzing equipment. 

Reliability. Reliability is indicated by the degree of 
constancy obtaining between repetitions of the same 
measurements. 

Reverberation Chamber. Chamber with hard, sound- 
reflecting walls. 

rms. Root-mean-square. 

rpm. Revolutions of the engine crankshaft per minute. 

RRU. Radio repeat unit. 

<r. Surface density of total structure. 

a Surface density of total structure, less Dural skin. 

<r m . Surface density of absorbing material alone. 

Schliere. Striation, due to local changes of index of 
refraction, made visible by schlieren (striation) 
method. 

Schlieren (Striation) Method. Optical method to 
allow visual observation of schlieren (striation). 

Semi-Insert Tip. Earphone socket designed to seal 
against the opening of the auditory canal. 


Sensation Level. The number of decibels a sound is 
above the threshold of audibility. 

Sensitivity of the Ear. See threshold. 

Simultaneous Range. A type of radio range transmis¬ 
sion which provides both range and voice signals on a 
single channel. Also called 5-tower range or TL 
range. 

SL. Spectrum level. The rms level of a sound of the 
continuous spectrum type, in bands one cycle wide. 
It is expressed in decibels relative to the standard 
reference level. 

Sone. A unit of loudness. It is defined as the loudness 
of a 1,000-cycle tone 40 decibels above the normal 
threshold. (The sone is a so-called “subjective” unit.) 

Sonic Velocity. Velocity of sound. 

Sound-Powered Telephone. A telephone system relying 
for operation only on the energy supplied by the 
talker’s voice. 

Span of Apprehension. The amount or complexity of 
perceptual material apprehended during a single 
brief presentation. 

Specific Flow Resistance. Flow resistance per unit 
thickness of sample. 

Spectrum. The spectrum of a sound of the continuous 
spectrum type is a plot of its spectrum level as a 
function of frequency. The spectrum of a wave con¬ 
taining a finite number of discrete components con¬ 
sists of a plot of the level of these components versus 
frequency. By extension, the spectrum of a wave 
containing both discrete and continuous frequency 
components is a combination of both types of spectra. 

SPI. Sonic Positioning Indicator. 

SPL. Sound pressure level. The sound pressure, in 
decibels, relative to the standard reference level of 
0.0002 dyne per square centimeter. 

SR. Sonic Receiver. 

STASI. Sonic True Airspeed Indicator. 

Static Canceling Circuits. Noise-reducing devices in 
radio receivers which operate on the principle that 
static (impluse noise) in adjacent channels can be 
used (by proper phase adjustment) to cancel out the 
impulse noise in the pass band of the receiver. 

Stimulation Deafness. A loss in hearing, usually 
temporary, resulting from exposure to intense sounds. 

Stream Velocity. Velocity at a point in the airstream. 

Subsonic Velocity. Velocity smaller than the sonic 
velocity. 

Supersonic Velocity. Velocity greater than the sonic 
velocity. 

Supra-Aural Socket. Earphone Socket designed to 
seal flat against the pinna. 

r. Transmission coefficient. 

T. Thickness of sound-absorbing material. 

Threshold. The threshold of audibility at a given fre¬ 
quency is the minimal value of sound pressure which 
produces an auditory sensation. (It can be expressed 
in terms of MAP or MAF.) The masked threshold is 
the minimal value of sound pressure necessary to 
hear the masked sound in the presence of a masking 



274 


GLOSSARY 


sound. The differential threshold (for frequency or 
intensity) is the minimal increment in the stimulus 
necessary to produce a just noticeable difference in 
sensation. Speech thresholds are obtained by adjust¬ 
ing the speech level (1) until the presence of speech 
is just noticeable although not intelligible ( threshold 
of detectability), (2) until the gist of the speech is 
understood, although individual sounds or words are 
not all heard ( threshold of perceptibility), or (3) 
until every sentence and phrase is understood without 
perceptible effort ( threshold of intelligibility). 

Throat Microphone. A microphone operated directly 
by the vibrations of the larynx during speech. 

Tilt. A type of frequency distortion. A circuit having 
a “tilted” frequency response that rises or falls at a 
constant rate over the entire range of frequencies 
under consideration. 

Tinnitus. An auditory disorder characterized by a 
ringing, buzzing, or rushing sound in the ear in the 


absence of any external stimulus. Occurs frequently 
following exposure to loud sounds. 

Tolerance Thresholds. The maximum sound pressure 
level which a listener can tolerate. Thresholds of dis¬ 
comfort, of tickle, and of pain can be distinguished. 

TPG. Timing Pulse Generator. 

TPR. Timing Pulse Receiver. 

U. S. Standard Atmosphere. Typical state of the 
atmosphere found in the continental U. S., defined in 
Publication No. 538, National Advisory Committee on 
Aeronautics. 

Validity. The extent to which a test or other measuring 
device measures what it purports to measure. 

Voice Tube. A tube forming an acoustic link between 
the mouth of a talker and the ear of a listener. 

Z. The effective level of a masking noise. Z is the 
difference in decibels between the pure-tone threshold 
in quiet and the total energy level in the critical 
band of noise frequencies. 



BIBLIOGRAPHY 


Numbers such as Div. 17-425-MI indicate that the document listed has been microfilmed and that its title 
appears in the microfilm index printed in a separate volume. For access to the index volume and to the 
microfilm, consult the Army or Navy agency listed on the reverse of the half-title page. 


Chapter 2 

1. “The Philips-Miller System of Sound Recording,” 
R. Veimieulen, Philips Technical Review, Vol. 1, 
1936, p. 107. 

2. “Loudness, Masking and Their Relation to the 
Hearing Process and the Problem of Noise Meas¬ 
urement,” Harvey Fletcher, The Journal of the 
Acoustical Society of America, Vol. 9, 1938, p. 
276. 

3. “The Absorption Coefficient Problem,” F. V. Hunt, 
The Journal of the Acoustical Society of America, 
Vol. 11, 1939, p. 39. 

4. “Flow of a Gas Through Porous Media,” J. L. 
Fowler and K. L. Hertel, Journal of Applied 
Physics, Vol. 11, 1940, p. 496. 

5. Materials and Techniques for Sound Control in 
Airplanes, Leo L. Beranek, R. L. Wallace, Jr., and 
others, OSRD 31, Excerpts from Progress Report 
of Project I, HEAL, Mar. 31, 1941. 

Div. 17-425-MI 

6. The Effects of Noise and Vibration on Psycho¬ 

motor Efficiency, S. S. Stevens, OSRD 32, Project 
II, HPAL, Mar. 31, 1941. Div. 17-435.22-MI 

7. “Official Bulletin No. VIII,” Acoustical Materials 
Association, Chicago, June 1941. 

8. Materials and Techniques for Sound Control in 

Airplanes, Leo L. Beranek, Rudolph H. Nichols, 
Jr., and others, OSRD 33, Project I, HEAL, June 
30, 1941. Div. 17-425-M2 

9. The Effects of Noise on Psychomotor Efficiency: I, 
Noise Reduction in Aircraft as Related to Com¬ 
munication, Annoyance and Aural Injury, II, S. S. 
Stevens, J. P. Egan, and others, OSRD 274, Re¬ 
port MHR-4, HPAL, Dec. 1, 1941. 

Div. 17-435.22-M2 

10. “American Standard Acoustical Terminology,” 
American Standards Association, Z24.1, 1942. 

11. Research on Ear Defenders, Norman A. Watson, 
OSRD 536, NDCrc-128, Service Project SC-4, Uni¬ 
versity of California at Los Angeles, Feb. 5, 1942. 

Div. 17-435.211-MI 

12. Design of an Automatic Octave Sound Analyzer 

and Recorder, H. Wayne Rudmose, Harold L. 
Ericson, and Hans F. Dienel, OSRD 969, OEMsr- 
658, HEAL, Nov. 21, 1942. Div. 17-425-M3 

13. Effect of Sound on Man and Means for Producing 

Such Sotmd, Harold Burris-Meyers, Theodore W. 
Forbes, and W. L. Woolf, OSRD 1255, OEMsr- 
197, Service Project CWS-18, Stevens Institute of 
Technology, Nov. 9, 1942. Div. 17-435.21-MI 

14. Collected Informal Reports on Sound Control in 
Airplanes, Leo L. Beranek, Rudolph H. Nichols, 


Jr., and others, OSRD 1323, OEMsr-658, HEAL, 
Apr. 10, 1943, Part 4. Div. 17-425-M4 

14a. Ibid., Part 10. 

15. Sound Levels Due to an Airplane Passing Over¬ 

head in Level Flight, Francis M. Wiener and 
Richard J. Marquis, OSRD 1404, OEMsr-658, 
HEAL, May 15, 1943. Div. 17-425-M5 

16. Temporary Deafness Following Exposure to Loud 

Tones and Noise, Hallowell Davis, Clifford T. 
Morgan, and others, OEMcmr-194, Harvard Medi¬ 
cal School, Sept. 30, 1943. Div. 17-435.21-M2 

17. Injury of the Inner Ear Produced by Exposure to 
Loud Tones (Supplementary Report), Joseph E. 
Hawkins, Jr., Moses H. Lurie, and Hallowell Davis, 
OEMcmr-194, Harvard Medical School, Dec. 31, 

1943. Div. 17-435.21-M3 

18. Principles of Sound Control in Airplanes, Leo L. 

Beranek, Rudolph H. Nichols, Jr., and others, 
OSRD 1543, Service Projects AN-C-93, AN-S-32, 
and AN-S-33, HEAL, 1944. Div. 17-425-M6 

18a. Ibid., p. 3. 

19. “Sound Waves in Rooms,” P. M. Morse and R. H. 
Bolt, Review of Modern Physics, Vol. 16, No. 2, 

1944, p. 75. 

20. The Relative Annoyance Produced by Various 
Bands of Noise, T. W. Reese and K. D. Kryter, 
Report IC-65, HPAL, Mar. 17, 1944. 

Div. 17-426-MI 

21. Sound Level Measurements in U.S. Military Air¬ 

planes, Volume 2, Hans F. Dienel, H. Wayne Rud¬ 
mose, and J. P. Lienesch, OSRD 3681, OEMsr-658, 
HEAL, June 1, 1944. Div. 17-425-M7 

22. Evaluation of Earplug Effectiveness, Report 2 on 
Project 257, AAF School of Aviation Medicine, 
July 1, 1944. 

23. Sound Absorbing Properties of Acoustical Ma¬ 

terials for Use Aboard Ships of the U.S. Navy, 
H. P. Sleeper, Jr., and Leo L. Beranek, OSRD 
4173, OEMsr-1240, Service Project N-109, HEAL, 
Oct. 1, 1944. Div. 17-423-MI 

24. Noise Reduction in LVT-U and LVT-A-U Amphibi¬ 

ous Tractors, Harold L. Ericson and Leo L. Ber¬ 
anek, OSRD 4249, OEMsr-1240, Service Project 
NS-315, HEAL, Oct. 9, 1944. Div. 17-421-M2 

25. Sound Level Measurements in U. S. Military Air- 
pla?ies, Volume 3, Hans F. Dienel, OSRD 4648, 
OEMsr-658, HEAL, Feb. 25, 1945. 

Div. 17-425-M8 

26. Noise Levels of Telegraphic Typewriters in Com¬ 

bat Information Center, J. P. Lienesch and 
Rudolph H. Nichols, Jr., Report CIR-48, HEAL, 
May 28, 1945. Div. 17-426-M2 

27. Reduction of Sound and Shock in Fortification 

275 


276 


BIBLIOGRAPHY 


Structures, H. P. Sleeper, Jr., and H. Wayne 
Rudmose, Report CIR-52, HEAL, Aug. 1, 1945. 

Div. 17-426-M3 

28. G. A. Miller and S. E. Mitchell, OSRD 5293, 

OEMsr-658, Service Project SC-118, HPAL, Aug. 
15, 1945. Div. 17-436.1-M2 

29. Sound Levels and Their Reduction in Type K Air¬ 

ships, Hans F. Dienel, OSRD 5512, OEMsr-658, 
HEAL, Aug. 25, 1945. Div. 17-425-M9 

30. The Quieting of Outboard Motors, Harold L. Eric- 
son, OSRD 6188, OEMsr-658, Service Project 
SAC-52, HEAL, Oct. 27, 1945. Div. 17-424-MI 

31. Sound Levels in a P-59B Jet Propelled Airplane, 
Hans F. Dienel and Rudolph H. Nichols, Jr., Re¬ 
port CIR-58, HEAL, Oct. 29, 1945. Div. 17-425-M10 

32. The Measurement of Acoustic Attenuation Char¬ 
acteristics of Sound-Proofing Materials for Air¬ 
craft, Hans F. Dienel, OEMsr-658 and N5ori-76, 
Project II, Report PNR-3, HPAL, Jan. 11, 1946. 

Div. 17-425-M11 


Chapter 3 

1. “The Auditory Masking of One Pure Tone by 
Another and Its Probable Relation to the Dy¬ 
namics of the Inner Ear,” R. L. Wegel and C. E. 
Lane, The Physical Review, Vol. 23, 1924, pp. 
266-285. 

2. “Differential Intensity Sensitivity of the Ear for 
Pure Tones,” R. R. Riesz, The Physical Review, 
Vol. 31, 1928, pp. 867-875. 

3. Speech and Hearing, Harvey Fletcher, D. Van 
Nostrand Co., 1929. 

4. “Differential Pitch Sensitivity of the Ear,” E. G. 
Shower and R. Biddulph, The Journal of the 
Acoustical Society of America, Vol. 3, 1931, pp. 
275-287. 

5. Hearing in Man and Animals, R. T. Beatty, G. 
Bell and Sons, 1932. 

6. “On Minimum Audible Sound Fields,” L. J. Sivian 
and S. D. White, The Journal of the Acoustical 
Society of America, Vol. 4, 1933, p. 288. 

7. “Loudness, Its Definition, Measurement and Calcu¬ 
lation,” H. Fletcher and W. A. Munson, The 
Journal of the Acoustical Society of America, Vol. 
5, 1933, pp. 106-107. 

8. Hearing, Its Psychology and Physiology, S. S. 
Stevens and H. Davis, John Wiley and Sons, 1938. 
8a. Ibid., pp. 110-127. 

9. “Auditory Patterns," Harvey Fletcher, Review of 
Modern Physics, Vol. 12, 1940, pp. 47-65. 

10. “The Relation of Pitch to Frequency, A Revised 
Scale,” S. S. Stevens and J. Volkmann, American 
Journal of Psychology, Vol. 53, July 1940, pp. 
329-353. 

11. “American Standard Acoustical Terminology,” 
American Standards Association, Z24.1, Mar. 20, 
1942, p. 7. 


12. Factors Governing the Intelligibility of Speech, 
N. R. French, Bell Telephone Laboratories, Sept. 
21, 1942. 

13. Effects of High Altitude on the Human Voice, 

H. Wayne Rudmose, Kenneth C. Clark, and others, 
OSRD 3106, OEMsr-658, HEAL, Jan. 30, 1944. 

Div. 17-435.11-M6 

14. Factors Involved in the Randomization of the 

Radar Pulse Repetition Frequency, OSRD 5124, 
OEMsr-658, Service Project NS-341, HPAL, June 
15, 1945. Div. 17-436.1-MI 

15. Auditory Tests for the Ability to Discriminate the 
Pitch and Loudness of Noises, J. E. Karlin, OSRD 
5294, OEMsr-658, Report MHR-124, HPAL, Aug. 

I, 1945. Div. 17-435.23-MI 

16. The Masking of Signals by Noise, S. S. Stevens, 
OSRD 5387, OEMsr-658, Service Projects NA-108, 
AN-10, and NS-341, HPAL, Oct. 1, 1945. 

Div. 17-436.1-M3 

16a. Ibid., Part II. 

17. The Pressure Distribution in the Auditory Canal 
in a Progressive Sound Field, Douglas A. Ross 
and Francis M. Wiener, OEMsr-658 and N5ori-76, 
Project II, Report PNR-5, HPAL, Dec. 1, 1945. 

Div. 17-435.21-M4 

18. Auditory Factors in the Discrimination of Radio 
Range Signals (Collected Informal Reports), J. P. 
Flynn, S. J. Goffard, and others, OSRD 6292, 
OEMsr-658, Service Project NA-108, Report MHR- 
104, HPAL, Dec. 31, 1945, Chap. 2. 

Div. 17-436.1-M4 


Chapter 4 

1. Speech and Hearing, Harvey Fletcher, D. Van 
Nostrand Co., 1929. 

la. Ibid., p. 6. 

lb. Ibid., p. 8. 

lc. Ibid., pp. 70-76. 

l d. Ibid., p. 279 ff. 

2. “Speech and Music,” J. C. Steinberg, Electrical 
Engineers’ Handbook, V. Electric Communication 
and Electronics (Third Edition), Pender and Mc- 
Ilwain, John Wiley and Sons, 1936, Sec. 9, p. 26. 

3. “Loudness, Masking and Their Relation to the 
Hearing Process and the Problem of Noise Meas¬ 
urement,” Harvey Fletcher, The Journal of the 
Acoustical Society of America, Vol. 9, 1938, p. 276. 

4. “Exploration of Pressure Field Around the Hu¬ 
man Head During Speech,” H. K. Dunn and D. W. 
Farnsworth, The Journal of the Acoustical Society 
of America, Vol. 10, 1939, p. 194. 

5. “Use of Thermocouple and Fluxmeter for Meas¬ 
urement of Average Power of Irregular Waves,” 
H. K. Dunn, The Review of Scientific Instruments, 
Vol. 10, 1939, p. 362. 

6. “The Carrier Nature of Speech,” Homer Dudley, 



BIBLIOGRAPHY 


277 


Bell System Technical Journal, Vol. 19, 1940, p. 
495. 

7. “Statistical Measurement of Conversational 
Speech,” H. K. Dunn and S. D. White, The Journal 
of the Acoustical Society of America, Vol. 11, 
1940, p. 284. 

7a. Ibid., p. 279. 

8. “A New Standard Volume indicator and Reference 
Level,” H. A. Chinn, D. K. Gannet, and R. M. 
Morris, Proceedings of the Institute of Radio En¬ 
gineers, Vol. 28, 1940, p. 1. 

9. Vocabularies for Military Communication in Noise, 
M. H. Abrams and J. E. Karlin, OSRD 1919, 
OEMsr-658, HPAL, Aug. 25, 1943, p. 80 ff. 

Div. 17-435.11-M3 

10. Effects of High Altitude on the Human Voice, 
H. Wayne Rudmose, Kenneth C. Clark, and others, 
OSRD 3106, OEMsr-658, HEAL, Jan. 30, 1944. 

Div. 17-435.11-M6 

11. “Visible Patterns of Sound,” R. K. Potter, Science, 
Vol. 102, 1945, p. 463. 


Chapter 5 

1. Relative Frequency of English Speech Sounds, 
Godfrey Dewey, Harvard University Press, 1923. 

2. “Articulation Testing Methods,” Harvey Fletcher 
and J. C. Steinberg, Bell System Technical Journal, 
Vol. VIII, No. 4, 1929. 

3. Speech and Hearing, Harvey Fletcher, D. Van 
Nostrand Co., 1929. 

4. Speech in Noise, A Study of the Factors Deter- 
mining Its Intelligibility, M. H. Abrams, S. J. 
Goffard, and others, OSRD 4023, OEMsr-658, Re¬ 
port MHR-81, HPAL, Sept. 1, 1944. 

Div. 17-435.11-M8 

5. Articulation Testing Methods, II, J. P. Egan, 
OSRD 3802, OEMsr-658, HPAL, Nov. 1, 1944. 

Div. 17-435.11-M9 

5a. Ibid., App. V. 


Chapter 6 

1. Collected Informal Communications, IC-32, A 

Vocabulary of Tactical Call Signs for Communi¬ 
cation in Noise, M. H. Abrams, J. E. Karlin, and 
others, OSRD 1571, OEMsr-658, Report MHR-39, 
HPAL, July 9, 1943. Div. 17-435.11-MI 

2. Vocabularies for Military Communication in Noise, 
M. H. Abrams and J. E. Karlin, OSRD 1919, 
OEMsr-658, HPAL, Aug. 25, 1943. 

Div. 17-435.11-M3 

2a. Ibid., pp. 49-56. 

The Audibility in Noise of a Proposed Fighter 


Director Vocabulary, M. H. Abrams and Joseph 
Miller, Report IC-57, HPAL, Dec. 31, 1943. 

Div. 17-435.11-M5 

4. Tactical Call Words, Technical Bulletin SIG 51, 
War Department, June 12, 1944. 


Chapter 7 

1. Speech and Hearing, Harvey Fletcher, D. Van 
Nostrand Co., 1929, p. 279 ff. 

2. “Effects of Distortion upon the Recognition of 
Speech Sounds,” J. C. Steinberg, The Journal of 
the Acoustical Society of America, Vol. 1, 1929, 
pp. 121-137. 

3. “Tests of Speech and Music Transmission,” J. C. 
Steinberg, Electrical Engineers’ Handbook, V. 
Electric Communication and Electronics (Third 
Edition), Pender and Mcllwain, John Wiley and 
Sons, 1936, Sec. 9, p. 25. 

4. “Transmission Features of the New Telephone 
Sets,” A. H. Inglis, Bell System Technical Journal, 
Vol. 17, 1938, pp. 358-380. 

5. Collected Informal Communications, Report IC-16, 

“The Effect on Articulation of Low Frequency 
Cutoff in Interphones,” J. P. Egan, D. R. Griffin, 
and others, OSRD 1572, OEMsr-658, Report MHR- 
40, HPAL, July 9, 1943. Div. 17-435.11-M2 

5a. Ibid., Report IC-12, “The Increased Intelligi¬ 
bility Gained by Extending the Response of an 
Earphone to 4,000 cps.” 

6. The Effects of Amplitude Distortion upon the In¬ 

telligibility of Speech, J. C. R. Licklider, OSRD 
4217, OEMsr-658, Service Project NA-108, HPAL, 
Nov. 15, 1944. Div. 17-435.11-M10 

7. On the Articulation Efficiency of Bands of Speech 
in Noise, J. P. Egan and Francis M. Wiener, 
OSRD 4872, OEMsr-658, Service Projects NA-108 
and NS-343, HEAL and HPAL, May 1, 1945. 

Div. 17-435.11-M11 


Chapter 8 

1. “The Auditory Masking of One Pure Tone by 
Another and Its Probable Relation to the Dy¬ 
namics of the Inner Ear,” R. L. Wegel and C. E. 
Lane, The Physical Review, Vol. 23, 1924, pp. 266- 
285. 

2. Studies on the Effect of Noise on Speech Com¬ 
munication, J. P. Egan, Joseph Miller, and others, 
OSRD 2038, OEMsr-658, HPAL, Nov. 25, 1943. 

Div. 17-435.11-M4 

3. G. A. Miller and S. E. Mitchell, OSRD 5293, 

OEMsr-658, Service Project SC-118, Report MHR- 
126, HPAL, Aug. 15, 1945. Div. 17-436.1-M2 

3a. Ibid., p. 67. 

3b. Ibid., Chap. IX. 


3. 



278 


BIBLIOGRAPHY 


4. The Masking of Signals by Noise, J. E. Hawkins, 
W. R. Garner, and G. A. Miller, OSRD 5387, 
OEMsr-658, HPAL, Oct. 1, 1945. Div. 17-436.1-M3 


Chapter 9 

1. Factors Governing the Intelligibility of Speech, 
N. R. French, Bell Telephone Laboratories, Sept. 
21, 1942. 

la. Ibid., p. 11. 

lb. Ibid., p. 19. 

2. The Performance of Communication Equipment 

in Noise, J. P. Egan, D. R. Griffin, and others, 
OSRD 901, OEMsr-658, Report MHR-21, HPAL, 
Oct. 1, 1942. Div. 17-436.31-MI 

3. The Articulation Efficiency of Magnetic and Dy¬ 

namic Earphones Used with Various Earphone 
Cushions in Noise, J. P. Egan, S. J. Goffard, and 
others, OSRD 1491, OEMsr-658, Report MHR-34, 
HPAL, June 15, 1943. Div. 17-436.31-M2 

4. Effects of High Altitude on the Human Voice, 
H. Wayne Rudmose, Kenneth C. Clark, and others, 
OSRD 3106, OEMsr-658, HEAL, Jan. 30, 1944. 

Div. 17-435.11-M6 

5. Articulation-Test Comparisons of Six Signal 
Corps Aircraft Interphones at Low and High Alti¬ 
tudes (appendix attached), K. D. Kryter, OSRD 
1974, OEMsr-658, ARL Memorandum Report 157, 
Report MHR-49, HPAL and HEAL, Mar. 1, 1944. 

Div. 17-436.31-M3 

6. Voltage Gain and Power-Output Capability Re¬ 
quirements for High-Altitude Interphone Ampli¬ 
fiers, J. C. R. Licklider, OSRD 1975, OEMsr-658, 
ARL Memorandum Report 158, Report MHR-50, 
HPAL and HEAL, Mar. 10, 1944. 

Div. 17-436.31-M4 

7. The Articulation Efficiency of Nine Carbon Micro¬ 
phones for Use at Low Altitudes, J. P. Egan, M. I. 
Stein, and G. G. Thompson, OSRD 3515, OEMsr- 
658, Report MHR-64, HPAL, June 1, 1944. 

Div. 17-436.31-M5 

8. Articulation Tests of Standard and Modified In¬ 
terphones Conducted During Flight at 5,000 and 
35,000 Feet (including appendices), J. C. R. Lick¬ 
lider and K. D. Kryter, OSRD 1976, OEMsr-658, 
ARL Memorandum Report 149, Report MHR-52, 
HPAL and HEAL, July 1, 1944. 

Div. 17-436.31-M7 

8a. Ibid., pp. 60-64. 

8b. Ibid., pp. 39-40. 

9. “Factors Governing the Intelligibility of Speech 
Sounds” (Abstract), N. R. French and J. C. Stein¬ 
berg, The Journal of the Acoustical Society of 
America, Vol. 17, 1945, p. 103. 

10. “Relation Between the Theory of Hearing and the 
Interpretation of Speech Sounds” (Abstract), 


W. A. Munson, The Journal of the Acoustical 
Society of America, Vol. 17, 1945, p. 103. 

11. The Articulation Efficiency of Certain American 

and Foreign Microphones, S. J. Goffard, J. Miller, 
and E. B. Newman, Report IC-127, HPAL, Aug. 
10, 1945. Div. 17-436.31-M9 

12. Measurements of Insulation and Sensitivity of 

Service Headsets, W. A. Shaw, OSRD 6113, 
OEMsr-658, Service Project NA-108, Report MHR- 
131, HPAL, Oct. 31, 1945. Div. 17-436.3-MI 

13. Problems of Voice Communication in Extremely 
High Ambient Noise: Landing Vehicles Tracked, 
Harold L. Ericson, Joseph Miller, and others, 
OSRD 5532, OEMsr-658, Service Projects NA-108 
and NS-315, HPAL and HEAL, Dec. 31, 1945. 

Div. 17-436.3-M2 

14. Audio Characteristics of Communication Equip¬ 
ment, OEMsr-658 and N5ori-76, Report PNR-6, 
HPAL, Feb. 1, 1946, Sec. F. Div. 17-436.3-M3 
14a. Ibid., p. 35. 


Chapter 10 

1. “A Voice and Ear for Telephone Measurements,” 
A. H. Inglis, C. H. G. Gray, and R. T. Jenkins, 
Bell System Technical Journal, Vol. 11, 1932, pp. 
293-317. 

2. “On Minimum Audible Sound Fields,” L. J. Sivian 
and S. D. White, The Journal of the Acoustical 
Society of America, Vol. 4, 1933, p. 288. 

3. “Some Data on a Room Designed for Free Field 
Measurements,” E. H. Bedell, The Journal of the 
Acoustical Society of America, Vol. 8, 1936, p. 118. 

4. “American Recommended Practice for the Cali¬ 
bration of Microphones,” American Standards As¬ 
sociation, Z24.4, 1938. 

5. “Absolute Measurement of Sound Without a Pri¬ 
mary Standard,” W. R. MacLean, The Journal of 
the Acoustical Society of America, Vol. 12, 1940, 
pp. 140-146. 

6. “Absolute Pressure Calibration of Microphones,” 
R. K. Cook, Jo%irnal of Research of the National 
Bureau Standards, Vol. 25, 1940, pp. 489-505; 
The Journal of the Acoustical Society of America, 
Vol. 12, 1941, pp. 415-420. 

7. “A Novel, Highly Effective Sound-Absorbing 
Arrangement and the Construction of a Dead 
Room,” E. Meyer, G. Buchmann, and A. Schoch, 
Akustische Zeitschrift, Vol. 5, 1940, p. 352. 
Abstract, R. W. Young, and O. H. Schuck, The 
Journal of the Acoustical Society of America, Vol. 
13, 1941, p. 191. 

8. “Acoustic Laboratory in the New RCA Labora¬ 
tories,” H. F. Olson, The Journal of the Acoustical 
Society of America, Vol. 15, 1943, p. 96. 

9. Collected Informal Reports on Interphone Equip- 



BIBLIOGRAPHY 


279 


10 . 


11 . 


12 . 


13. 


14. 


15. 


16. 


17. 


18. 


19. 


20. 


merit, Leo L. Beranek, Francis M. Wiener, and 
others, OSRD 1324, OEMsr-658, HEAL, Apr. 12, 
1943. Div. 17-436.321-M2 

The Electronic Generation of Airplane Noise for 
Use in Testing and Training, E. B. Newman and 
S. S. Stevens, OSRD 1445, OEMsr-658, Report 
MHR-33, HPAL, May 25, 1943. Div. 17-412-MI 
Interphone Equipment, Rudolph H. Nichols, Jr., 
A. S. Filler, and others, OSRD 1524, OEMsr-658, 
Army Project SC-48, Reports C-l to C-5, HEAL, 
June 1, 1943. Div. 17-436.321-MI 

The Articulation Efficiency of Magnetic and Dy¬ 
namic Earphones Used with Various Earphone 
Cushions in Noise, J. P. Egan, S. J. Goffard, and 
others, OSRD 1491, OEMsr-658, Report MHR-34, 
HPAL, June 15, 1943, pp. 33-35. 

Div. 17-436.31-M2 
Interphone Equipment, Rudolph H. Nichols, Jr., 
A. S. Filler, and others, OSRD 1545, OEMsr-658, 
Army Project SC-48, Reports C-6 to C-10, HEAL, 
July 5, 1943. Div. 17-436.321-MI 

Collected Informal Communications on Articula¬ 
tion Tests of Interphone Equipment, J. P. Egan, 
D. R. Griffin, and others, OSRD 1572, OEMsr-658, 
Report MHR-40, HPAL, July 9, 1943. 

Div. 17-435.11-M2 
A Modified Tank Crash Helmet for Use with a 
Separate Telephone Headset, B. M. Flynn and 
John Volkmann, Report IC-41, HPAL, Aug. 15, 
1943. Div. 17-435.241-MI 

Design, Operation and Calibration of the Sound 
Pressure Meter, Francis M. Wiener, Rudolph H. 
Nichols, Jr., and others, OSRD 1817, OEMsr-658, 
HEAL, Sept. 25, 1943. Div. 17-436.323-MI 

16a. Ibid., p. 8. 

Interphone Equipment, Rudolph H. Nichols, Jr., 
A. S. Filler, and others, OSRD 1927, OEMsr-658, 
Army Project SC-48, Reports C-ll to C-20, HEAL, 
Oct. 20, 1943. Div. 17-436.321-MI 

Microphone and Headset Studies, J. P. Egan, 
J. E. P. Libby, and others, OSRD 2037, OEMsr- 
658, Service Project SC-48, HPAL, Nov. 20, 1943. 

Div. 17-436.322-MI 


Response Characteristics of Interphone Equip¬ 
ment (Revision IV to be inserted in OSRD 687), 
Francis M. Wiener, H. Wayne Rudmose, and 
others, OSRD 3105, OEMsr-658, HEAL, Jan. 1, 


1944. 




Div. 

19a. 

Ibid., 

Sec. 

B-II, pp. 80-105. 

19b. 

Ibid., 

Sec. 

B-VIII. 


19c. 

Ibid., 

Sec. 

B-VII. 


19d. 

Ibid., 

Sec. 

B-IV. 


19e. 

Ibid., 

Sec. 

D-II, App. 

B. 

19f. 

Ibid., 

Sec. 

D-III. 


19g. 

Ibid., 

Sec. 

D-IV, D-V, 

App. A. 

19h. 

Ibid., 

Part 

E-II. 



Articulation Tests of A-U and XA-13 Oxygen 
Masks at Sea Level and at 35,000 Feet, Douglas 
A. Ross, E. B. Ginsburg, Jr., and others, Report 


IC-55, HPAL, June 5, 1944. Div. 17-436.31-M6 

21. Interphone Equipment, Rudolph H. Nichols, Jr., 

A. S. Filler, and others, OSRD 3682, OEMsr-658, 
Army Project SC-48, Reports C-21 to C-27, HEAL, 
June 6, 1944. Div. 17-436.321-MI 

22. Interphone Equipment, Rudolph H. Nichols, Jr., 

A. S. Filler, and others, OSRD 3683, OEMsr-658, 
Army Project SC-48, Report C-28, HEAL, June 
10, 1944. Div. 17-436.321-MI 

23. The Impairment of Acoustic Transmission Due to 

Items of Army Clothing for Cold-Weather Wear 
[Part I], W. A. Shaw, Report IC-76, HPAL, June 
28, 1944. Div. 17-435.242-MI 

24. Interphone Equipment, Rudolph H. Nichols, Jr., 

A. S. Filler, and others, OSRD 3905, OEMsr-658, 
Army Project SC-48, Reports C-29 and C-31, 
HEAL, Aug. 2, 1944. Div. 17-436.321-MI 

25. Measurements of Acoustic Insulation in Three 
Types of Aviation Helmets, W. A. Shaw, Report 
IC-89, HPAL, Aug. 7, 1944. Div. 17-435.241-M2 

26. AN-N-7 Noise Measuring Equipment, Army-Navy 
Aeronautical Specification, Oct. 5, 1944. 

27. The Articulation Efficiency of Three Types of 
Headsets Proposed for Use by the Canadian 
Ground Forces, J. P. Egan, W. A. Shaw, and 
others, Report IC-92, HPAL, Nov. 15, 1944. 

Div. 17-436.31-M8 

28. A Comparison of the Acoustic Insulation Afforded 
by Certain Aviation Helmets When Worn with 
Oxygen Masks and Goggles, W. A. Shaw, Report 
IC-90, HPAL, Nov. 20, 1944. 

Div. 17-435.241-M3 

29. Evaluation of Hearing Aids, Rudolph H. Nichols, 

Jr., Richard J. Marquis, and others, OSRD 4666, 
OEMsr-658, Service Project AN-10, HEAL, May 
1, 1945, Part I, p. 93. Div. 17-435.212-MI 

30. Impairment of Acoustic Transmission Due to 
Items of Army Clothing for Cold-Weather Wear, 
[Part] II, W. A. Shaw, D. E. Yates, and E. B. 
Newman, Report IC-118, HPAL, Apr. 20, 1945. 

Div. 17-435.242-M2 

31. An Analysis of the Acoustic Insulation and 

Acoustic Sensitivity of Certain U. S. Navy Sound- 
Powered Headsets, W. A. Shaw, Report IC-123, 
HPAL, July 1, 1945. Div. 17-435.243-M3 

32. The Design and Construction of Anechoic Sound 
Chambers, Leo L. Beranek, H. P. Sleeper, Jr., and 
others, OSRD 4190, OEMsr-658, Service Projects 
NA-108 and AC-9, HEAL, Oct. 15, 1945. 

Div. 17-436.323-M3 

32a. Ibid., pp. 95, 98. 

32b. Ibid., pp. 10-13, 32-48. 

33. Collected Reports on the Performance of Special 

Communication Equipment, Rudolph H. Nichols, 
Jr., R. L. Wallace, Jr., and others, OSRD 6311, 
OEMsr-658, Service Projects NA-108 and NS-343, 
HEAL, Oct. 24, 1945. Div. 17-436.324-M3 

34. Microphones, Earphone Cushions and Handsets 
for Special Applications, P. S. Veneklasen and 



280 


BIBLIOGRAPHY 


Joseph Miller, OSRD 6310, OEMsr-658, Service 
Project NA-108, HEAL, Oct. 22, 1945, Sec. 2. 

Div. 17-436.324-M2 

35. Measurements of Insulation and Sensitivity of 

Service Headsets, W. A. Shaw, OSRD 6113, 
OEMsr-658, Service Project NA-108, HPAL, Oct. 
31, 1945. Div. 17-436.3-MI 

35a. Ibid., p. 40. 

36. The Response of Certain Earphones on the Ear 
and on Closed Couplers, Francis M. Wiener and 
A. S. Filler, OEMsr-658 and N5ori-76, Project II, 
Report PNR-2, HPAL, Dec. 1, 1945. 

Div. 17-436.322-M3 

37. On the Technique of Absolute Pressure Calibration 

of Condenser Microphones by the Reciprocity 
Method, Alfred L. DiMattia and Francis M. 
Wiener, OEMsr-658 and N5ori-76, Report PNR-4, 
HPAL, Dec. 10, 1945. Div. 17-436.324-M4 

38. Audio Characteristics of Communication Equip¬ 
ment, OEMsr-658 and N5ori-76, Project II, Report 
PNR-6, HPAL, Feb. 1, 1946, Section G-l. 

Div. 17-436.3-M3 

38a. Ibid., Sec. G-2. 

38b. Ibid., Sec. M. 

38c. Ibid., Sec. J. 


Chapter 11 

1. The Acoustic Design of Earphone Sockets for Hel¬ 

mets and Headsets, D. R. Griffin, John Volkmann, 
and others, OSRD 826, Report MHR-19, HPAL, 
Aug. 20, 1942. Div. 17-435.243-MI 

2. Acoustical Considerations in the Design of an 

Oxygen Mask, Thomas E. Cay wood and Leo L. 
Beranek, OSRD 952, OEMsr-658, HEAL, Oct. 19, 
1942. Div. 17-435.12-MI 

3. Thin Headphone for Use Under M-l Helmet, W. W. 

Weedfall, OSRD 1805, OEMsr-658, Report CIC-23, 
HEAL, Sept. 15, 1943. Div. 17-436.46-MI 

4. Speech and Sound Transmission Through Gas 

Masks, J. P. Egan, E. B. Ginsburg, and others, 
OSRD 1816, OEMsr-658, HPAL and HEAL, Sept. 
20, 1943, Part I. Div. 17-435.12-M2 

4a. Ibid., Part II. 

5. Response Characteristics of Interphone Equip¬ 

ment (Revision IV to be inserted in OSRD 687), 
Francis M. Wiener, H. Wayne Rudmose, and 
others, OSRD 3105, OEMsr-658, HEAL, Jan. 1, 
1944, Part B-VII. Div. 17-436.321-M3 

6. An Evaluation of the Acoustic Insulation and the 
Acoustic Sensitivity of the Harvard Design 8-C 
Earphone Socket, W. A. Shaw, John Volkmann, 
and others, Report IC-116, HPAL, Apr. 7, 1945. 

Div. 17-435.243-M2 

7. Microphones, Earphone Cushions and Handsets for 
Special Applications, P. S. Veneklasen and Joseph 


Miller, OSRD 6310, OEMsr-658, Service Project 
NA-108, HEAL, Oct. 22, 1945, Part 4. 

Div. 17-436.324-M2 

7a. Ibid., Part 5. 

7b. Ibid., Parts 1, 2. 

8. Reaction of Small Enclosures on the Human Voice, 
Transmission of Speech Through Gas Masks, C. T. 
Morrow, OSRD 6309, OEMsr-658, Service Project 
CWS-28, HEAL, Oct. 26, 1945. 

Div. 17-435.12-M3 

8a. Ibid., p. 27. 

8b. Ibid., p. 85. 

9. Measurements of Insulation and Sensitivity of 
Service Headsets, W. A. Shaw, OSRD 6113, 
OEMsr-658, Service Project NA-108, Report 
MHR-131, HPAL, Oct. 31, 1945. Div. 17-436.3-MI 


Chapter 12 

1. Audio Frequency Factors in Portable Radio Equip¬ 
ment, I; Redesign of SCR-274N and AT A Modula¬ 
tors, II, Harold L. Ericson, Alfred L. DiMattia, 
and others, OSRD 1528, OEMsr-658, Reports I to 
III, HEAL, June 28, 1943. Div. 17-436.44-MI 

2. Submarine Voice Tube Systems, R. L. Wallace, 
Jr., Thomas E. Caywood, and others, OSRD 1889, 
OEMsr-658, HEAL, Oct. 15, 1943. 

Div. 17-436.42-MI 

3. Performance of U.S. Navy Sound-Powered Tele¬ 

phones, Francis M. Wiener, W. G. Wiklund, and 
others, OSRD 3789, OEMsr-658 and OEMsr-1240, 
Service Projects N-109 and OFS-Nr-1, HEAL and 
HPAL, June 15, 1944. Div. 17-436.43-MI 

4. Comparison of the Intelligibility Afforded by 
Type M and Type O Sound-Powered Headsets, 
J. P. Egan, F. M. Wiener, and Joseph Miller, Re¬ 
port IC-108, HPAL and HEAL, Feb. 20, 1945. 

Div. 17-436.322-M2 

5. Comparison of the Intelligibility Afforded by Type 
M and Type O Sound-Powered Headsets, [Part] 
II, J. P. Egan, Francis M. Wiener, and others, 
Report IC-119, HPAL and HEAL, Apr. 23, 1945. 

Div. 17-436.322-M2 

6. On the Articulation Efficiency of Bands of Speech 
in Noise, J. P. Egan and Francis M. Wiener, OSRD 
4872, OEMsr-658, Service Projects NA-108 and 
NS-343, HPAL and HEAL, May 1, 1945, pp. 2-3. 

Div. 17-435.11-M11 

7. An Analysis of the Acoustic Insulation and Acous¬ 

tic Sensitivity of Certain U. S. Navy Sound-Pow¬ 
ered Headsets, W. A. Shaw, Report IC-123, H1JAL, 
July 1, 1945. Div. 17-435.243-M3 

8. A Communication System for Use in Shallow- 
Water Diving, R. L. Wallace, Jr., and C. T. Mor¬ 
row, OSRD 5375, OEMsr-658, Service Project 
NS-343, HEAL, July 25, 1945. Div. 17-436.41-MI 




BIBLIOGRAPHY 


281 


9. Comparison of the Intelligibility Afforded by Vari¬ 
ous Modifications of Type M and Type O Sound- 
Powered Telephones, [Part] IV, J. P. Egan and 
Francis M. Wiener, Report IC-134, HPAL and 
HEAL, Sept. 15, 1945. Div. 17-436.43-M2 

10. Collected Informal Reports on the Performance of 
Special Communication Equipment, Rudolph H. 
Nichols, Jr., R. L. Wallace, Jr., and others, OSRD 
6311, OEMsr-658, Service Projects NA-108 and 
NS-343, HEAL, Oct. 24, 1945. Div. 17-436.324-M3 

11. Battle Telephone Adapter Kits, Revised, H. Wayne 
Rudmose, Report CIR-60, HEAL, Oct. 30, 1945. 

Div. 17-436.43-M3 

12. The Response of Certain Earphones on the Ear 
and on Closed Couplers, Francis M. Wiener and 
A. S. Filler, OEMsr-658 and N5ori-76, Project II, 
Report PNR-2, HPAL, Dec. 1, 1945. 

Div. 17-436.322-M3 


Chapter 13 

1. An Electronic Device to Simulate Atmospheric 
Static, S. S. Stevens, R. L. Wallace, Jr., and 
others, Report IC-75, HPAL, May 29, 1944. 

Div. 17-412-M2 

2. “Reducing Radio Noise,” Carlton Wasmansdorff, 
Electronic Industries, July 1944. 

3. Articulation Tests of the Wasmansdorff Noise 
Peak Limiter, J. C. R. Licklider, M. I. Stein, and 
S. S. Stevens, Report IC-84, HPAL, Sept. 12, 1944. 

Div. 17-438.2-MI 

4. The Advantages of Clipping the Peaks of Speech 

Waves Prior to Radio Transmission, K. D. Kryter, 
M. I. Stein, and S. S. Stevens, Report IC-83, 
HPAL, Oct. 10, 1944. Div. 17-438.2-M2 

5. The Combined Effects of Clipping the Peaks of 

Speech Waves in an ATB Transmitter and Limit¬ 
ing Static Peaks in an ARB Receiver, K. D. Kry¬ 
ter, S. J. Goffard, and S. S. Stevens, Report IC-93, 
HPAL, Mar. 15, 1945. Div. 17-438.2-M3 

6. Field Tests of Pre-Modidation Clipping in the 

Transmitter of a Type 19 Wireless Set, J. C. R. 
Licklider and E. B. Newman, Report IC-121, 
HPAL, May 8, 1945. Div. 17-438.2-M4 

7. A Premodulation Clipper Unit for Voice-Com¬ 

munication Transmitters, J. C. R. Licklider, G. A. 
Roberts, and S. S. Stevens, Report IC-100, HPAL, 
June 30, 1945. Div. 17-438.2-M5 

8. Generation of Radio Frequency Noise by Means 
of Short Pidses, C. J. Mullin and H. Wayne Rud¬ 
mose, OEMsr-658 and N5ori-76, Project II, Re¬ 
port PNR-8, HPAL, Oct. 28, 1945. Div. 17-413-MI 

9. A Two-Carrier System for Radio Communication, 
W. J. Cunningham and J. C. R. Licklider, OSRD 
6112, Service Projects NA-108 and NS-365, Re¬ 
port MHR-135, HPAL and HCCR, Oct. 31, 1945. 

Div. 17-438.2-M6 


10. Investigation of Principles Underlying the Maxi¬ 

mizing of Communication Intelligibilty (Terminal 
Report), R. P. Lett and K. S. Kunz, OSRD 6280, 
OEMsr-1441, Service Projects NS-365 and NS- 
108 (Division 13, Part III), Problem 4, HCCR, 
Dec. 1, 1945. Div. 17-630-MI 

11. Auditory Factors in the Discrimination of Radio 
Range Signals, Collected Informal Reports, J. P. 
Flynn, S. J. Goffard, and others, OSRD 6292, 
OEMsr-658, Service Project NA-108, Report 
MHR-104, HPAL, Dec. 31, 1945. Div. 17-436.1-M4 
11a. Ibid., Chaps. Ill, IV, and V. 

lib. Ibid., Chap. VI. 

11c. Ibid., Chap. VII. 

12. The Performance of Counter-Modulation and 
Static-Canceling Circidts in Aircraft Radio Re¬ 
ceiver AN/ARR-15, J. C. R. Licklider and S. J. 
Goffard, OEMsr-658 and N5ori-76, Project II, 
Report PNR-9, HPAL, Mar. 1, 1946. 

Div. 17-438.1-MI 

12a. Ibid., p. 8. 

13. Effects of Static on Radio Range Performance, 
Laboratory Tests of the Improvement Provided by 
Noise-Reducing Circuits, S. J. Goffard and J. C. R. 
Licklider, OEMsr-658 and N5ori-76, Project II, 
Report PNR-10, HPAL, Mar. 21, 1946. 

Div. 17-438.1-M2 

13a. Ibid., pp. 24-27. 

13b. Ibid., pp. 28-32. 


Chapter 14 

1. The Problem of Selecting and Training Personnel 

for Communication in Intense Noise, OSRD 987, 
OEMsr-658, Report MHR-27, HPAL, Nov. 10, 

1942. Div. 17-441-MI 

2. A Speech Interview for the Selection of Telephone 

Talkers, OSRD 1769, OEMsr-830, Service Project 
N-109, Applied Psychology Panel, Report 1, The 
Psychological Corp., August 1943, p. 18. 

Div. 17-441.1-MI 

3. A Memory Test for Digits Phono graphically Re¬ 

corded for Group Administration, M. H. Abrams, 
James F. Curtis, and others, Report IC-38, HPAL, 
July 16, 1944. Div. 17-442-MI 

4. Speech in Noise, A Study of the Factors Determin¬ 
ing Its Intelligibility, M. H. Abrams, S. J. Gof¬ 
fard, and others, OSRD 4023, OEMsr-658, Report 
MHR-81, HPAL, Sept. 1, 1944. Div. 17-435.11-M8 
4a. Ibid., Report IC-67, “Subjective Ratings of the 
Intelligibility of Talkers in Noise.” 

5. Auditory Tests of the Ability to Hear Speech in 

Noise, J. E. Karlin, M. H. Abrams, and others, 
OSRD 3516, OEMsr-658, Report MHR-66, HPAL, 
Sept. 1, 1944. Div. 17-435.11-M7 



282 


BIBLIOGRAPHY 


6. Operating Instructions for Psycho-Acoustic Labo¬ 
ratory Training Interphone Type 202, OEMsr-658, 
Report MI-15, HPAL, Oct. 1, 1944. 

Div. 17-441.2-MI 

7. Ability to Hear Sentences in Tones, Manual of 
Instructions for Auditory Test No. 13, Report 
IC-97, HPAL, Dec. 28, 1944. Div. 17-442-M2 

8. The Relation between the Ability to Listen in 

Noise and Ability to Listen in Stepped Tones, J. E. 
Karlin and S. S. Stevens, Report IC-98, HPAL, 
Jan. 20, 1945. Div. 17-442-M3 

9. A Project for Standardizing Submarine Phrase¬ 
ology and Developing a Training Program in Sub¬ 
marine Voice Communication, M. H. Abrams, 
Louis A. Mallory, and others, OSRD 4795, OEMsr- 
658, OEMsr-830, and OEMsr-1128, Service Project 
N-118, Report P57/1421 (Joint Report of NDRC 
6.1, 17.3, APP), Harvard University, The Psy¬ 
chological Corp., and CUDWR-NLL, Feb. 28, 1945. 

Div. 17-441-M2 


Chapter 15 

1. Hearing, Its Psychology and Physiology, S. S. 
Stevens and H. Davis, John Wiley and Sons, 1938, 
pp. 133-135. 

2. “Selective Amplification in Hearing Aids,” N. A. 
Watson and V. O. Knudsen, The Journal of the 
Acoustical Society of America, Vol. 11, 1940, pp. 
406-416. 

3. “Methods for Measuring the Performance of 
Hearing Aids,” F. F. Romanow, The Journal of 
the Acoustical Society of America, Vol. 13, 1942, 
p. 294. 

3a. Ibid., p. 297. 

4. Threshold of Hearing for Words, Manual of In¬ 

structions for Auditory Test No. 9, Report IC-73, 
HPAL, May 20, 1944. Div. 17-435.25-MI 

5. Threshold of Hearing for Sentences, Manual of 
Instructions for Atiditory Test No. 12, Report 
IC-96, HPAL, Dec. 20, 1944. Div. 17-435.25-M2 

6. “Tentative Code for Measurement of Performance 
of Hearing Aids,” The Journal of the Acoustical 
Society of America, Vol. 17, 1945, pp. 144-150. 

7. Evaluation of Hearing Aids, Rudolph H. Nichols, 
Jr., Richard J. Marquis, and others, OSRD 4666, 
OEMsr-658, Service Project AN-10, HEAL and 
HPAL, May 1, 1945, Part I and Supplement. 

Div. 17-435.212-MI 

7a. Ibid., Part II. 

8. Report of Sound Conference, June 14 and 15, 
1945, sponsored by Research Division, Bureau of 
Medicine and Surgery, Medical Research Depart¬ 
ment, U. S. Submarine Base, New London. 

9. Tolerance for Pure Tones and Speech in Normal 
and Hard-of-Hearing Ears, OSRD 6303, OEMsr- 
1201, Service Project AN-10, Central Institute 


for the Deaf, July 31, 1946. Div. 17-435.21-M5 

10. Selection of Hearing Aids, Hallowell Davis and 
Douglas A. Ross, OEMsr-658 and N5ori-76, Proj¬ 
ect II, Report PNR-7, HPAL, Dec. 31, 1945. 

Div. 17-435.212-M2 


Chapter 16 

1. Acoustic Direction Finding for Aircraft Intercep¬ 

tion, including Information on a Microphone for 
Operation in High Wind Velocities, R. L. Wallace, 
Jr., and Eugene Ennis, OSRD 317, HEAL, Dec. 
15, 1941. Div. 17-437.3-MI 

2. Sonic Position Indicating Equipment for Use in 
Blind Flying of Towed Gliders, R. L. Wallace, Jr., 
Harold L. Ericson, and others, OSRD 3097, 
OEMsr-658, HEAL, Feb. 1, 1944. Div. 17-437.2-MI 
2a. Ibid., p. 53. 

2b. Ibid., p. 45. 


Chapter 17 

1. “Sound Waves in the Atmosphere,” E. A. Milne, 
The London, Edinburgh and Dublin Philosophical 
Magazine and Journal of Science, Vol. 42, 1921, 
p. 96. 

2. “Aircraft Speed Instruments,” K. Hilding Beij, 
National Advisory Committee for Aeronautics, 
Report 420, 1932. 

3. “Velocity of Sound in Air,” H. C. Hardy, D. Tel¬ 
fair, and W. H. Pielemeier, The Journal of the 
Acoustical Society of America, Vol. 13, No. 3, Jan¬ 
uary 1942, p. 226. 

4. Sonic True Airspeed Indicator, Volume I, Julian 
Eisenstein, Kenneth C. Clark, and Francis D. 
Carlson, OSRD 5369, OEMsr-658, Service Project 
NA-170, HEAL, Aug. 27, 1945, p. 10. 

Div. 17-436.323-M2 

5. Sonic True Airspeed Indicator, Volume II, Julian 
Eisenstein, Kenneth C. Clark, and Francis D. 
Carlson, OSRD 5370, OEMsr-658, Service Project 
NA-170, HEAL, Sept. 1, 1945, p. 7. 

Div. 17-436.323-M2 

5a. Ibid., p. 33. 

5b. Ibid., App. II. 


Chapter 18 

1. “True Blind Flight,” Luis De Florez, Journal 
Aeronautical Science, Vol. 3, 1936, pp. 168-170. 

2. Flybar [or] Flying by Auditory Reference, Theo¬ 
dore W. Forbes, W. R. Garner, and J. G. Howard, 
OSRD 5123, OEMsr-658, Service Project NA-108, 
Report MHR-107, HPAL, June 1, 1945. 

Div. 17-437.1-MI 



BIBLIOGRAPHY 


283 


Chapter 19 

1. A Method of Indicating Time on Voice Record¬ 

ings, Francis M. Wiener, R. A. Walker, and Ken¬ 
neth C. Clark, OSRD 4189, OEMsr-1240, Service 
Projects N-109 and OFS-Nr-1, HEAL, Oct. 25, 
1944. Div. 17-436.324-MI 

la. Ibid., p. 7. 

2. C. J. Mullin, R. G. Huebschen, and others, OSRD 


4974, OEMsr-658, Service Project NS-343, HEAL, 
Apr. 13, 1945. Div. 17-620-MI 

2a. Ibid., Part B, p. 1. 

2b. Ibid., Part B, p. 8. 

3. The Radio Repeat Unit, An Application of Mag¬ 
netic Recording, R. H. Carson and A. E. Sander¬ 
son, OSRD 6070, OEMsr-658, Service Project NS- 
343, HEAL, Oct. 1, 1945. Div. 17-500-M5 

3a. Ibid., p. 17. 

3b. Ibid., p. 12. 



OSRD APPOINTEES 

Division 17 


Chiefs 

George R. Harrison 
Paul E. Klopsteg 
Deputy Chiefs 
E. A. Eckhardt 
Paul E. Klopsteg 


Miles J. Martin 
Francis L. Yost 

0. S. Duffendack 
Theodore Dunham, Jr. 
E. A. Eckhardt 
Harvey Fletcher 


Technical Aides 

Clark Goodman 

John A. Hornbeck (WOC) 

Members 

William E. Forsythe 
George R. Harrison 
Herbert E. Ives 
Brian O’Brien 

Melville I. Stein 


Charles B. Bazzoni 


Gioacchino Failla 


Section 17.1 

Chief 

E. A. Eckhardt 
Deputy Chief 
Herbert E. Bragg 
Spec. Asst, to Chief 
John A. Hornbeck 
Technical Aide 
Herbert E. Bragg 
Members 
J. M. Cork 

Section 17.2 

Chief 

Melville I. Stein 
Technical Aide 
George E. Beggs 

Members 

C. H. Willis 


Semi J. Begun 


J. C. Hubbard 


Section 17.3 

Chief 

Harvey Fletcher 
Spec. Asst, to Chief 
William S. Gorton 
L. J. Sivian 
Acting Chief 
P. M. Morse 
Technical Aides 

William S. Gorton (WOC) 

Members 

Hallowell Davis 
Floyd A. Firestone 

E. C. Wente 


Clifford Morgan 

Vern 0. Knudsen 
S. Smith Stevens 


CONTRACT NUMBERS, CONTRACTORS AND SUBJECTS OF CONTRACTS 


Contract Name and Address 

Number 0 f Contractor 

NDCrc-52 National Academy of Sciences 

Washington, D. C. 


NDCrc-79 Harvard University 

Cambridge, Massachusetts 


NDCrc-110 The Western Electric Company, 

porated 

New York, New York 


Subject 


Studies and investigations in connection 
with noise levels, the effects of noise and 
methods and materials for noise reduction 
in combat vehicles. 

Studies and experimental investigations in 
connection with physiological and psycho¬ 
logical effects of and the development of 
protection against sounds. 

Incor- Studies and experimental investigations in 
connection with the development of sound 
sources and arrangement of such sources 
for producing high directivity of the 
sound beam radiated. 


NDCrc-128 The Regents of the University of 

California 

Berkeley, California 


Studies and experimental investigations in 
connection with the development of sound 
sources. 


OEMsr-38 National Academy of Sciences 


OEMsr-39 National Academy of Sciences 


Studies and experimental investigations in 
connection with the feasibility of de¬ 
veloping a directional acoustic microphone 
for plane following. 

Studies and experimental investigations of 
noise levels, effects of noise, methods and 
materials for noise reduction in combat 
vehicles, and such other aspects of air 
space or water borne sound including 
problems of underwater submarine de¬ 
tection as may be requested in writing by 
the Contracting Officer prior to July 1, 
1942. 


OEMsr-119 Harvard University 

Cambridge, Massachusetts 


Studies and experimental investigations in 
connection with physiological and psycho¬ 
logical effects of and the development of 
protection against sounds. 


OEMsr-146 Western Electric Company, Incorporated Studies and experimental investigations in 

New York, New York connection with (i) the development of 

sound sources and arrangement of such 
sources for producing high directivity of 
the sound beam radiated, including de¬ 
velopment of a siren type of sound pro¬ 
jector and the testing of the projector de¬ 
veloped, (ii) the design and development 
of an air raid warning siren, smaller than 
the high directive siren but employing the 
principles thereof, and (iii) the delivery 
and testing, in various parts of the United 
States as the Contracting Officer or his 
authorized representative shall direct, of 
ten (10) models of such sirens as may be 
developed hereunder. 


285 









CONTRACT NUMBERS, CONTRACTORS AND SUBJECT OF CONTRACTS ( Continued) 


Contract 

Number 

Name and Address 
of Contractor 

Subject 

OEMsr-172 

American Locomotive Company 

Auburn, New York 

To supply one (1) centrifugal air com¬ 
pressor, consisting of a ninety-five (95) 


horsepower Ford engine, complete with 
starter, fuel tank, fan, radiator, etc., a 
gear box and gears, and a blower with 
air filter, said compressor to be capable 
of delivering two thousand five hundred 
(2,500) cubic feet per minute at a pres¬ 
sure of five (5) pounds. 


OEMsr-176 Western Electric Company, Incorporated Studies and experimental investigations in 

New York, New York connection with the development of meth¬ 

ods and apparatus for reducing the static 
coming from the radio set and the noise 
coming from the engines of the plane dur¬ 
ing the reception of speech by airplane 
pilots. 


OEMsr-197 


The Trustees of the Stevens Institute of 
Technology 
Hoboken, New Jersey 


Studies and experimental investigations in 
connection with the determination of the 
effect of noises on man and the develop¬ 
ment of means and methods for producing 
such noises. 


OEMsr-658 President and Fellows of Harvard 

Cambridge, Massachusetts 


College Instrumental and psychological studies and 
experimental investigations in connection 
with (i) sound control in combat vehicles 
and adequacy and efficiency of interphone 
equipment, (ii) acoustical investigations 
involving similar equipments and tech¬ 
niques, (iii) acoustical methods of posi¬ 
tion determination of gliders towed by 
planes, (iv) tests of samples and new 
models of microphones, head phones and 
interphone equipment, (v) development of 
tests and methods for selecting and train¬ 
ing voice communications personnel, (vi) 
efficiency and adequacy of standard and 
experimental hearing aids for service in¬ 
curred deafness among military personnel, 
(vii) the development of a sonic method 
for the indication of the true airspeed of 
aircraft, and (viii) interior shipboard 
communications and Combat Communica¬ 
tion Centers and the development of de¬ 
vices employed therein. 


OEMsr-849 Chrysler Corporation 

Detroit, Michigan 


Studies and experimental investigations in 
connection with the development of a 
siren-type intense sound source suitable 
for airplane mounting. 


286 







CONTRACT NUMBERS, CONTRACTORS AND SUBJECT OF CONTRACTS ( Continued) 


Contract 

Number 

Name and Address 
of Contractor 

Subject 

OEMsr-908 

Western Electric Company, Incorporated 

New York, New York 

Conduct studies and experimental investiga¬ 
tions in connection with the (i) develop¬ 
ment of a loud-speaking system capable 
of being placed manually and suitable for 
use at sea or on land for purposes of 
deception and decoy and for conveying 
intelligence, (ii) development, for further 
manufacture, of ten improved and lighter 
weight loud-speaking systems for use in 
military operations (junior heaters), and 
(iii) development of three loud-speaking 
systems suitable for installation in elec¬ 
trically driven torpedoes. 

OEMsr-1178 

Mine Safety Appliances Company 

Pittsburgh, Pennsylvania 

Studies and experimental investigations in 
connection with the production of (i) one 
four-cavity mould for small tips, (ii) two 
four-cavity moulds for medium tips, (iii) 
one four-cavity mould for large tips, and 
(iv) 500 Harvintips of assorted sizes. 

OEMsr-1201 

Central Institute for the Deaf, a Missouri 
Corporation 

St. Louis, Missouri 

Studies and experimental investigations in 
connection with (i) the development of 
instruments and methods for aiding im¬ 
paired hearing incurred in military serv¬ 
ice, and (ii) the development of tests for 
the evaluation of such instruments and 
methods when used with partially deaf¬ 
ened individuals. 

OEMsr-1210 

Pennsylvania State College 

State College, Pennsylvania 

Studies and experimental investigations in 
connection with (i) the development and 
testing of equipment and methods for 
utilizing high frequency sound waves for 
communication in warfare, and (ii) the 
supersonic acoustics of jungle terrains. 

OEMsr-1230 

General Electric Company 

Schenectady, New York 

Studies and experimental investigations in 
connection with the development of three 
electrically driven torpedoes containing 
suitable loudspeaking systems which will 
perform specified functions at the end of 
a prescribed run of the torpedo. 

OEMsr-1240 

President and Fellows of Harvard College 
Cambridge, Massachusetts 

Instrumental and experimental investiga¬ 
tions in connection with (i) interior com¬ 
munication facilities, instruments and 
recording devices for use on ships of the 
Navy, (ii) surveys and studies of efficient 
means for displaying radar and other in¬ 
formation to ship’s personnel, (iii) meth¬ 
ods and materials for acoustically quieting 
and air conditioning operations rooms, 
and (iv) the construction of experimental 
equipment to demonstrate and test the 
improvements enumerated herein. 


287 









CONTRACT NUMBERS, CONTRACTORS AND SUBJECT OF CONTRACTS ( Continued ) 


Contract 

Number 

Name and Address 
of Contractor 

SiLbject 

OEMsr-1300 

National Organ Supply Company 

Erie, Pennsylvania 

Studies and experimental investigations in 
connection with (i) the production of 
1250 pairs of small, 2500 pairs of medium, 
and 1250 pairs of large V-51R ear 
wardens, together with the required 
number of applicators and cases, and the 
moulds required for their production, and 
(ii) the use of thermo-plastic materials 
for ear defenders and other aural devices 
used in communication equipment. 

OEMsr-1335 

The Trustees of Rutgers College in New 
Jersey 

New Brunswick, New Jersey 

Studies and experimental investigations in 
connection with the acoustics of jungle 
terrains. 


288 





SERVICE PROJECT NUMBERS 



The projects listed below were transmitted to the Office of the 

Executive Secretary, OSRD, from the War or Navy Department 
through either the War Department Liaison Officer for NDRC 
or the Office of Research and Inventions (formerly the Co¬ 
ordinator of Research and Development), Navy Department. 

Service 

Project 

Number 

Title 

AC-9 

AC-43 

AN-10 

CE-36.02 

CE-39 

CWS-18 

Army Projects 

Soundproof Material. 

Acoustic Device for Determining the Position of a Glider with Respect to a Towing Airplane. 
Research in Problems of Rehabilitation of the Deaf. 

Reduction of Sound and Shock in Fortification Structures. 

Weapons Simulators. 

Determination of the Effect of Noise on Men and Development of Means and Methods of Produc¬ 
ing Such Noises. 

CWS-28 
CWS-28 Ext. 
OD-110 

SC-4* 

SC-47 

SC-48 

SC-105 

SC-118 

Study of Acoustical Properties of Gas Masks. 

Films of Better Acoustic Properties for Diaphragm Masks. 

High-Intensity Expendable Sound Source. 

Sound Research. 

Deceptive Sounds. 

Microphone and Headset Studies. 

Acoustics of the Jungle. 

Psychological Factors in Communication Jamming. 


* This woi’k is not discussed in Division 17 STR. For details the reader is referred to the NDRC Bi-Monthly 
Summaries. The project is listed here for completeness of project information. 


N-109® 

N-118 Ext. 

Navy Projects 

Assistance in Selection and Training of Personnel Using Voice Communication Systems. 

Assistance in Program of Classification and Training of Telephone Talkers at Submarine Base, 
New London. 

NA-108 

NA-108 Ext. 
NA-108 Ext. 

Speech Intelligibility of Aircraft Intercommunication Equipment. 

Assistance in the Preparation of an Operational Use Manual. 

Determination of the Optimum Time Constant of the Audio Limiter Circuit of the AN/ARC-2 
Equipment. 

NA-118 

NA-170 

NA-183 b 

NR-104 

NR-107 

NS-191 

NS-262 

NS-315 

NS-315 Ext. 
NS-341 

NS-343 

NS-343 Ext. 

NS-365 C 

SAC-52 

Noise-Making Device for Naval Aircraft. 

Sonic True Airspeed Indicator. 

Sonic Control of Guided Missiles. 

Heater. 

Development of Supersonic Whistle and Receiver. 

Flat-Type Headset Receiver. 

Sound Transmission in Air. 

Noise Reduction in LVTs. 

Construction of Two Experimental Audio Amplifiers for use in LVTs. 

Countermeasures to Enemy Detection of Radar. 

Interior Communications and Associated Equipment. 

Study of the Conversion of a VG Type Radar Repeater. 

Investigations of Principles Underlying the Maximizing Communication Intelligibility. 

Quieting of Outboard Motors. 


a Division 17 cooperated in work on this project, but N-109 was not formally assigned to it. 
b Division 17 was associated with this project in an advisory capacity only. The project was not formally 
assigned to Division 17. 

c This project was assigned to Divisions 13 and 17 in cooperation. 


289 












INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 
For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


A-14 oxygen mask with micro¬ 
phone, 133, 143-146 
Absorption coefficient, acoustic ma¬ 
terials, 18, 27, 32 
Acoustic airspeed indicators 
see Sonic airspeed indicator 
Acoustic direction-finding device, 
233 

Acoustic materials, 19-31 
absorbent material, 22-23 
absorption coefficient, 18, 27, 32 
absorptive structures, 21 
acoustic impedance, 27 
attenuation properties, 19-22 
efficiency index, 21, 28, 31 
flow resistance of material, 22-28 
merit factor, 21 

requirements for sound control, 
19 

sound absorption measurement, 
27-28 

sound attenuation measurement, 
23-27 

Acoustic positioning indicator 
see Sonic positioning indicator 
AD (altitude decrement), 63, 138, 
162-163 

Adapters for microphones, 192 
AI (articulation index), 124-126 
Air conduction hearing aid, 218 
Aircraft interception, sonic direc¬ 
tion finding, 233 

Airplane communication, 105-108 
Airplane noise 

see Noise in airplanes 
Airplane piloting by auditory 
signals, 255-260 
automatic annunciator, 257-258 
Link trainer, 255-257 
psychological principles of hear¬ 
ing, 259-260 
tone signals, 255-257 
Airplane pursuit meter, 38, 255 
Airspeed indicator 

see Sonic airspeed indicator 
Airspeed studies 

see Wind tunnel, airspeed studies 
Alphabet, phonetic, for military 
messages, 81-82 

Altitude, effect on communication, 
105-108 

Altitude decrement (sound pres¬ 
sure ratio), 63, 138, 162-163 
AM-26/AIC, interphone amplifier, 

135 


Ambient noise, measurements 
see Noise, ambient, measurement 
American Telephone and Telegraph 
Company, 82 

Amplifier for interphone systems, 
134-135 

Amplitude limiters, radio, 198-200 
evaluation, 200 
peak clipping, 198 
premodulation clipping, 198 
ANB-H-1 magnetic earphone 
couplers for testing earphone, 
157-162 

cutoff frequency, 98 
in Harvard handset, 178-179 
in headset, 136 

ANB-H-1A dynamic earphone, 136, 
195 

ANB-M-C1 carbon microphone 
artificial voice calibrations, 149 
high altitude tests, 105-108 
in handset, 178 

mounted in an enclosure, 133, 
146-149 

response characteristics, 163 
Anechoic chamber, 167-173 
applications, 42, 48, 58, 152 
normal absorption coefficient, 168 
pressure-reflection coefficient, 168 
Annunciator, automatic, 257-258 
Armstrong-Mallock ear defenders, 
43 

Articulation index, 124-126 
Articulation tests, 69-80 

see also Communication, special 
vocabularies 

administration of test items, 73 
articulation score, 69 
item difficulty, 70-73 
on equipment in ambient noise 
fields, 76 

qualifications of administrator 
and testees, 73 

test material and methods, 70-73, 
77-79, 106, 117 
validity of tests, 70-74 
“Artificial ears,” for testing equip¬ 
ment, 157-162 
Ataxiameter, 38 
Audiogram, 35, 52 
Audio-spectrometer, 58-63, 142-146 
amplitude distribution of speech, 
61 

measurement of noise at micro¬ 
phone, 123 


speech at high altitudes, 62, 138 
speech spectrum analysis, 58 
testing microphones, 142-146 
Auditory signals for instrument 
flying, 255-260 

automatic annunciator, 257-258 
Link trainer, 255-257 
psychological principles of hear¬ 
ing, 259-260 
tone signals, 255-257 
Auditory tests, 212-214, 216-218, 
231, 232 

ability to hear in presence of 
noise, 213 

clinical auditory tests, 232 
electro-acoustic equipment, 231, 
232 

intelligence as a factor, 212-214 
objectives, 216 

pure-tone audiometric tests, 216 
recorded listening tests, 56 
tests of tolerance thresholds, 231 
validity of tests, 212-214 
Aural protective devices, 40-45 
acoustic insulation, 42 
blast protection, 45 
design, 41 

effect on reception of speech, 44 
material, 42 

range of frequency of insulation, 
44 

types, 40-42 

Automatic annunciator, 257-258 

Band-pass systems in communica¬ 
tion equipment, 101-104 
Basilar membrane in ear, 47 
Baum’s Mega eardrum protectors, 
42 

BC-212 interphone amplifier, 135 
BC-347C interphone amplifier, 135 
Bernoulli wave front equation, 253 
Binaural threshold, 50 

Cancellation circuit, radio receiver, 
201-202 

Carbon hearing aid, 218 
Carbon microphones, 106, 128-134, 
163 

hand-held microphones, 128-129 
microphones mounted in an en¬ 
closure, 133-134 

noise-canceling microphones, 130- 
131 

throat microphones, 131-133 


291 


292 


INDEX 


Central Institute for the Deaf 
aural thresholds, 230 
hearing aids, 231-232 
Chemical Warfare Board, 46 
Circuits, electronic 

cancellation circuit, radio re¬ 
ceiver, 201-202 
clipping circuits, 90-96 
countermodulations circuit, 200- 
202 

flip-flop circuit, 245 
Circumaural earphone sockets, 174- 
175 

Clipping circuits, communication 
equipment 

center clipping, 87-88, 94-95 
peak clipping, 90-96 
Close-talking microphone, 92 
Communication, signal interfer¬ 
ence, 109-118 

alternating interferences, 118 
condition for maximum inter¬ 
ference, 118 

effect on listener efficiency, 111 
intermittent noise, 110 
modulated pulses, 115 
noise interference, 116-118 
random noise, 116 
tonal interference, 111-118 
vocal interference, 118 
Communication, special vocabu¬ 
laries, 81-85 

alphabetic equivalents, 81-82 
alternative pronunciations of 
numerals, 82-83 

effect of number of syllables on 
intelligibility, 84 
highly intelligible words, 83-84 
telephone directory names, 83 
use of phonetic alphabet, 81-82 
Communication at high altitudes, 
105-108 

Communication equipment, design 
considerations, 86-118 
ambient noise, 17-28, 75 
amplitude distortion, 86-96, 104- 
108 

amplitude distribution of speech, 
61 

band-pass systems, 97-104 
clipping circuits, 94-95 
earphone response, 152-155 
frequency distortion, 96-108 
human ear, 47-57 
interphone components, 135, 174- 
187 

linear rectification, 95 
microphone response, 142-152 


recommendations for future re¬ 
search, 108 
resonant peaks, 103 
signal interference, 109-118 
speech spectrum, 58-61 
tilted response characteristics, 99 
Communication personnel, selec¬ 
tion, 208-215 
listeners, 212-214 
speech intelligibility factors, 
210-212 

speech interview, 208-210 
talkers, 208-212 

test of listening ability, 212-214 
testing equipment, 214 
training of personnel, 214-215 
Communication systems 

see Voice communication systems 
Condenser microphone, 48-50, 92, 
142-167 

calibration, 164-167 
calibration of loudspeaker for 
microphone testing, 149 
combined with couplers, 158 
free field sound pressure meas¬ 
urement, 142 

in sound pressure meter, 163 
measurement of earphone insula¬ 
tion, 156 

sound pressure distribution in 
auditory canal, 49-50 
Coordinated serial pursuit meter, 
38, 255 

Countermodulation circuit, radio 
receiver, 200, 201 

Couplers (artificial ears), 157-162 
CTD (per cent combination tone 
distortion), 89, 90 
CTE-49015 earphones, 136 
CW-49507 headset, 194 
CW-49507A headset, 179 
CW-51066 microphone, 194 

D-ll noise shield, microphone, 179 
D-17 noise shield, microphone, 133, 
180 

D-18 noise shield, microphone, 143 
D-25 noise shield, microphone, 182 
Dead-reckoning tracer, 268-269 
Deafness, temporary, 34-35 
Direction finding, sonic, 233 
Directivity pattern of ship motor 
noise, 12, 13 

Diving, communication system, 
194-196 

DRT (dead-reckoning tracer), 268- 
269 

Dural on airplanes, 17-19 
Dynamic earphones, 136, 152-155, 
195, 250 


Dynamic microphone, 132 

E17R8 speech transmitter, 185-186 
Ear, human, 47-57 

see also Auditory tests 
description, 47 
masking of tones, 52-53 
maximum amplification, 49 
pitch and loudness, 51 
psychological principles of hear¬ 
ing, 259-260 

resonance in ear canal, 48 
sensitivity, 50, 55-57 
sound-pressure distribution, 47- 
50 

Ear, protective devices 

see Aural protective devices 
Ear defenders, 43 
Earphones 

for interphone systems, 135-138 
for sonic array, 250 
hearing aids, 216-232 
miniature, 179 
real-ear response, 123 
resonance, 103 

response measuring techniques, 
151-162 

sockets, 137, 174-177 
special applications, 178-179 
testing, 151-163 

types, 98, 99, 136, 151-163, 195 
Earplugs 

see Aural protective devices 
Ears, artificial, for testing equip¬ 
ment, 157-162 

Efficiency index of acoustic ma¬ 
terials, 28-31 

Eglin Field interphone tests, 120 
Electro-acoustic equipment for 
clinical auditory tests, 231, 
232 

Five tower range system, 204 
Flip-flop circuit, striation appara¬ 
tus, 245 

Flow resistance of acoustic ma¬ 
terials, 22-28 

“Flybar” (flying by auditory refer¬ 
ence) , 255-260 

automatic annunciator, 257-258 
Link trainer, 255-257 
psychological principles of hear¬ 
ing, 259-260 
tone signals, 255-257 
Formulas 

acoustic gain of hearing aids, 
219 

articulation index, 124 
flow resistance of porous ma¬ 
terial, 23 



INDEX 


293 


merit factor of acoustic ma¬ 
terials, 21 

orthotelephonic gain of a sys¬ 
tem, 123 

sound attenuation of acoustic 
materials, 20 

transmission coefficient of sound, 
18 

Fort Bragg, North Carolina, 46 
Frequency distortion in communi¬ 
cation equipment, 96-108 

Gas masks, effect on speech, 184- 
186 

Generator, static, 197-198 
Generators for testing micro¬ 
phones, 146 

H-4/AR headset, 136, 157 
H-16/U headset, 137, 153, 177 
Harvard University 

anechoic chamber, 167-173 
circumaural earphone sockets, 
174-175 

handset, 178-179 
noise shields, 143, 179-182 
static-canceling circuit, 200 
wind tunnel studies, 241-248 
Harvintip earphone socket, 177 
HD (per cent harmonic distortion), 
89, 90 
Headsets 

interphone, 135-138 
Marine, 194 

types, 135-138, 152-156, 177 
Headsets, testing, 151-163 
frequency response, 151-156 
probe-tube technique, 152 
real-ear response, 151-152 
threshold technique, 155 
use of artificial ear, 157-162 
Hearing aids, 216-232 

acoustic gain measurement, 219 
air conduction type, 218 
carbon aid, 218 

compression amplification, 230 
design objectives, 229-231 
electro-acoustic measurements, 
217-225 

evaluation of performance, 217- 
228 

field test results, 221-225 
laboratory test results, 221 
measuring hearing loss, 216-217 
peak clipping, 95, 230 
psycho-acoustic measurements, 
225-227 

vacuum tube aid, 218 
Hearing aids, selection, 228-230 
audiogram fitting, 228-229 


maximum articulation score, 228 
maximum operating range, 229 
principle of selective amplifica¬ 
tion, 228 

quality preferences, 229 
required acoustic gain, 230 
Hearing loss, temporary, 34-35 
Hearing process 
see Ear, human 
Hearing tests 

see Auditory tests 
High-pass systems in communica¬ 
tion equipment, 97-98 
HS-23 headset, 136, 157 
HS-30 headset, 136, 153 
HS-33 headset, 136, 157 

Indicator, sonic airspeed 
see Sonic airspeed indicator 
Instrument flying, auditory signals, 
255-260 

automatic annunciator, 257-258 
Link trainer, 255-257 
psychological principles of hear¬ 
ing, 259-260 
tone signals, 255-257 
Instruments for ships 

see Shipboard special instru¬ 
ments 

International Telecommunications 
Convention, 82 
Interphone, 119-141 
amplifier, 134-135 
articulation index, 124-126 
at altitude, 138-141 
calibration of microphones, 164- 
167 

components of system, 127-135 
earphone testing, 152-162 
electrical response of network, 
123 

equivalent free-field sound pres¬ 
sure, 122 

factors determining performance, 

121, 122 

for rating and training talkers, 
214-215 

microphone testing, 142-152 
orthotelephonic gain measure¬ 
ments, 122-124 

overall response of system, 122- 
123 

performance in noise, 120-127 
prediction of performance, 124- 
127 

sound-powered telephone, 188- 
193 

sources of noise pickup, 120-121 
speech to noise ratio, 126-128 
Interphone design, 174-187 


boom mounting for microphones, 
186 

earphone sockets and insert tips, 
174-177 

gas masks, 184-186 
handsets and earphones for 
special applications, 178-179 
noise shields, 179-182 
oxygen masks, 182-183 

Joint Radio Board 
couplers, 158 

frequency response of micro¬ 
phones, 148 

noise pickup in microphones, 150 

Korogel, 150 

Link trainer, 255-257 
Listening tests 
see Auditory tests 
Loudspeaker for testing micro¬ 
phones, 148 

M-5/UR microphone, 130, 186 
M-6/UR microphone, 146, 192 
Mach numbers, 243 
MAF (minimum audible free-field 
pressure at eardrum), 50 
Magnetic earphones, 99, 136, 157- 
162, 178 

Magnetic microphones, 77, 92, ISO- 
138, 239 

Magnetic tape for radio repeat 
unit, 261-265 

Magnetic throat microphone, 77 
Maguire Industries, Inc., static¬ 
canceling circuit, 200 
Mallock-Armstrong ear defenders, 
43 

MAP (minimum audible pressure 
at eardrum), 50 
Mask microphone, 133, 178 
Masking, aural, 52-53 
application to radar, 54 
critical bandwidth, 52 
effect of tonal duration, 53 
masking audiogram, 52 
of complex tones, 54 
of tones by noise, 52, 76 
pure tone masking, 109 
threshold of masking, 52 
Materials, acoustic 
see Acoustic materials 
MC-253 microphone 

effect of peak clipping, 92 
in sonic positioning indicator, 
239, 250 

MC-253A microphone 

artificial voice calibrations, 150 




294 


INDEX 


response characteristics, 163 
use of oxygen mask, 138 
Mel (unit of pitch), 51 
MF of acoustic materials (merit 
factor), 21 
Microphones 
adapters, 192 
boom mounting, 186 
carbon, 106, 128-134, 163 
close-talking, 92 

condenser microphones, 48-50, 92, 
142-167 
dynamic, 132 

for shallow-water diving com¬ 
munication, 194 
hand held microphone, 128 
magnetic, 77, 92, 130-138, 239 
noise shields, 133, 143, 179-182 
noise-canceling microphones, 130, 
144, 192, 194, 195 
oxygen masks, 133, 143, 182-183 
real-voice response, 123 
throat microphones, 92, 131-133 
Microphones, testing, 142-173 
altitude decrement, 162 
artificial voice, 146-148 
free-field calibration, 166-167 
frequency response, 142-146 
human voice, 142-173 
noise generators, 146 
noise pickup, 146-151 
nonlinear distortion, 151 
pressure calibrations, 168 
random sound field, 144 
real-voice response, 142 
sound-pressure meter, 163 
speech-to-noise ratio, 144 
Military communication vocabu¬ 
laries 

see Communication, special vo¬ 
cabularies 

Military messages, phonetic alpha¬ 
bet, 81-82 

Miller film recorder, 15 
Monaural threshold, 50 
Motor coordination, effect of noise 
see Psychomotor efficiency, effect 
of noise 

Motor coordination apparatus, 36 
MSA ear defenders, 43 
MTS-1 Harvard handset, 178 
MX-41/AR earphone socket, 137 

NAF-38610 headset, 136 
NAF 68304 beam filter, 205 
NDRC ear wardens, 43 
Nelson’s ear stoppers, 43 
Neoprene ear plugs, 42, 46 


New London Submarine School, 
training of communication 
personnel, 214-215 
Nods noise mufflers, 43 
Noise, ambient, measurement, 4-17 
aircraft factories, 17 
airplane noise, 7-13 
automatic sound analyzer, 6 
effect of altitude on noise level, 8 
machine gun noise level, 9 
methods and apparatus, 4-7 
noise spectrum, 5 
ship noise, 10 

tank and landing vehicle noise, 
10 

Noise, ambient, reduction, 17-32 
acoustic materials, 19-23 
recommendations, 31-32 
sound absorption, 19 
sound attenuation, 18 
test methods and apparatus, 23- 
28 

Noise, effect on personnel 

see Psychomotor efficiency, effect 
of noise 

Noise analysis apparatus, 4-7 
Noise in airplanes, 7-41 
altitude effect, 8 
effect of vibration on visual 
acuity, 40 

effect on auditory organ, 34-35 
interference with communication, 
36 

jet-propelled airplane, 8 
noise spectrum, 7-10, 32 
prediction of noise level, 28-31 
reduction of noise, 22, 31, 42 
Noise interference in communica¬ 
tion 

see Communication, signal inter¬ 
ference 

Noise peak limiters, radio, 93, 198- 
200 

evaluation, 200 
peak clipping, 198 
premodulation clipping, 198 
Noise reduction in radio 
see Radio noise reduction 
Noise shields for microphones, 133, 
143, 179-182 
Noise spectrum 
effect of shape, 34 
maximum interference with 
speech, 118 
of airplanes, 7-10, 32 
of bombers, 29 
of submarines, 12 
of tanks, 10 

Noise-canceling microphones, 130, 
146, 192, 194, 195 


Non-linear distortion, 86-96, 104- 
108 

Numerals, alternative pronuncia¬ 
tion, 82-83 

Olygo noise absorbers, 43 
Orthotelephonic gain (OG), 96- 
101, 121-124 
concept, 96 
measurement, 122-124 
of sound powered telephones, 189 
Ossicles of ear, 47 
Oxygen masks with microphones, 
133, 182-183 

PBR-10, permoflux earphones, 111, 
152, 250 

Peak clipping circuits, 90-95 
in hearing aids, 95, 230 
in radio receivers, 198 
peak clipped speech, 90-95 
Permoflux Coi'poration, miniature 
earphone, 179 

Permoflux PDR-10 earphone, 111, 
152, 250 

Personnel selection, communication 
equipment 

see Communication personnel, 
selection 

Phon (unit of loudness), 51 
Phonetic alphabet for military 
messages, 82 

Pitch modulator, radio, 207 
Pitch test, 56 

Pitot-static airspeed indicator, 240 
Point-source projector for use on 
dead reckoning tracers, 268- 
269 

Positioning indicator, sonic 

see Sonic positioning indicator 
PRF (pulse repetition frequency), 
54,113 

Probe tube for testing earphones, 
48, 152 

Psychological Corporation, auditory 
test, 214 

Psychomotor efficiency, effect of 
noise, 32-40 
card sorting, 39 
coding test, 39 

coordinated serial pursuit, 38-39 
coordinated serial reaction-time, 
36 

fast-speed pursuit rotor, 39 
hearing effects, 34-36 
physical effects, 34-40 
serial disjunctive reaction-time, 
39 

visual effects, 38-40 



INDEX 


295 


Public-address systems, military 
application, 193 

Pulse repetition frequency, 113 
Pursuit meter, airplane, 39, 255 

R-14 earphones, 136 
R-30 earphones, 137 
Radar applications of masking of 
tones, 54 

Radio noise reduction, 197-207 
amplitude limiters, 198-200 
beam filter for radio range re¬ 
ceivers, 205 

effect upon reception of speech, 
207 

fluctuation noise, 197 
noise-peak limiters, 93, 198-200, 
206 

pitch modulator, 207 
radio range signal expander, 207 
recommendations, 207 
static, 197-198, 200-204, 206 
Radio range system, 57, 204-207 
A and N signals, 204 
beam filters, 205 
noise limiters, 206 
pitch modulator, 207 
signal expander, 207 
simultaneous range, 204 
static canceling circuit, 206 
static density, 205 
Radio receiver, cancellation cir¬ 
cuit, 201-202 

Radio repeat unit, 261-265 
Random noise, 116 
Range systems 

see Radio range systems 
RCA word list, 82 
Reciprocity method of calibrating 
microphones, 164 

Recommendations for future re¬ 
search 

aircraft noise reduction, 22, 31, 
42 

noise reduction in combat ve¬ 
hicles, 31-32 

radio noise reduction, 207 
ship noise reduction, 31 
sound-powered telephones, 192- 
193 

speech signals in communication 
equipment, 108 
Relays, voice-operated, 146 
Reverberation chamber, 27 
RRU (radio repeat unit), 261-265 

Saccadic eye movements, effect of 
noise, 38 

Schlieren method, wind tunnel 
studies, 243-245 


coincidence system, 243-244 
electronic circuits, 244-245 
high speed light soux-ce, 244-245 
spark gap sound source, 244-245 
Seashore pitch test, 56 
Sensation level 
of noise, 117 
of sound, 51-53 
of speech, 126 

Sepco safety ear protectors, 43 
Serial disjunctive reaction-time, 
effect of noise, 39 
Shipboard special instruments, 
261-269 

dead-reckoning tracer, 268-269 
lighting of plotting and display 
surfaces, 269 

point-source projector, 269 
radio repeat unit, 261-265 
time indication on voice record¬ 
ings, 265-268 
Side-tone monitoring, 65 
Signal expander, 207 
Signal interference 

see Communication, signal inter¬ 
ference 

Simultaneous range system, 204 
Siren, wind-driven, for sonic direc¬ 
tion finding, 237 

SL of speech (spectrum level), 58, 
121 

SMR ear stoppers, 43 
Sockets for earphones, 174-177 
circumaural, 174-175 
comparison of sockets, 137 
dual-seal, 177 
insert tips, 176 

Sone (psychological loudness unit), 

51 

Sonic airspeed indicator, 240-254 
advantages, 241 
applications, 241 
definition of true airspeed, 240 
flight tests, 251-253 
for Navy blimp, 248-253 
low speed wind tunnel, 241 
pitot-static indicator, 240 
sonic array, 250 

sound propagation in moving air, 
241-248 


Sonic positioning indicator, 233- 
239 

aerodynamic noise at gliders, 235 
microphone location, 235 
operation, 234-235 
receiver, 237-239 
sound source, 237 
transmission of acoustic signals, 
235-237 

Sound analyzer, automatic, 6 
Sound absorbing materials 
see Acoustic materials 
Sound absorption, 27-28 
Sound attenuation, 19-27 
measurement, 23-27 
weight law, 20 
Sound control, 4-46 
aircraft factories, 17 
aural protective devices, 40-45 
measurement of ambient noise, 
4-17 

noise level in tanks, 10 
prediction of aircraft noise 
levels, 28-31 

recommendations for ship noise 
reduction, 12, 13, 31-32 
reduction of airplane noise, 22-23 
reduction of ambient noise, 17-28 
Sound intensity level, definition, 51 
Sound level measurement 

see Noise, ambient, measurement 
Sound level reduction, 17-32 
absorption of sound, 18 
acoustic materials, 19-23 
attenuation of sound, 18 
recommendations, 31-32 
test methods and apparatus, 23- 
28 

Sound systems 

see Voice communication systems 
Sound transmission coefficient, 18, 
30 

Sound-powered telephones, 188-193 
advantages and limitations, 191- 
192 

bandwidth, 189 
efficiency, 189 
orthotelephonic gain, 189 
performance in noise, 189-192 
suggested improvements, 192-193 
use, 188 


stream velocity, 240 Sound-pressure levels, 99 

summary of operational chail>ECL^8'Sl^^H ure meter > 163 

teristics, 242 J}y T75pectTTmr4evel of speech, 58, 121 

theory of operation, 241 sties, human, 58- 

velocity distribution in wind 68 

tunnels, 253-254 OCi i anwlttwle distribution, 61 

Sonic direction finding, aircraft arnrmmdon measurements, 69- 

interception, 233 t-. » 80, 101-103, 124-126 

Defense memo 2 IAug ust 1960 

library op congress 



296 


INDEX 


audio-spectrometer, 58 
effect of altitude, 62, 105-108 
effect of noise, 210-212 
factors affecting intelligibility, 
75-77, 90-95, 210-212 
low frequencies of speech, 98 
peak factor, 65, 99 
phonemic rate of speech, 58 
recognition of speech sounds, 66- 
68 

speech spectrum, 58-61 
unvoiced sounds, 58 
volume indicator, 60 
vowel triangle, 60 
SPI 

see Sonic positioning indicator 
SPL (sound-pressure level), 99 
SR (sonic receiver), 234, 237-239 
STASI 

see Sonic airspeed indicator 
Static generators, 197-198 
Static reduction in radio com¬ 
munication, 197-198, 200-204, 
206 

cancellation (W Pass) circuit, 
201-202 

countermodulation circuit, 200, 
201 

density of static, 197, 205 
fluctuation noise, 197-198 
impulse noise, 197 
r-f signal-to-static ratio, 204 
static generator, 197-198 
two-carrier system, 202-204 
Submarine voice tubes, 196 

T-17 hand held microphone, 128- 
134 

T-30 throat microphone, 131-134 
T-30-P throat microphone, 92 


T-38 hand held microphone, 128 
T-45 noise canceling microphone, 
130 

Tank noise factors, 10 
TH-37 headset, 157 
Thresholds 
binaural, 50 

hearing tolerance, 230, 231 
monaural, 50 
speech articulation, 79 
visual, 38 

Throat microphones, 92, 130-133 
Tinnitus, noise sensation, 35 
Transmission coefficient of sound, 
18, 30 

Two-carrier receiver, 202-204 
Tympanic membrane, ear, 47 

University of California, 41 

V-29 ear warden, 41, 43 
V-51R ear warden, 42-44 
Vacuum tube hearing aid, 218 
Vinylite for ear plugs, 42, 46 
Visual acuity, 32, 40 
Visual effects of noise, 38-40 
Vocabularies for military com¬ 
munication 

see Communication, special vo¬ 
cabularies 

Voice communication 

see Speech characteristics, 
human 

Voice communication systems, 188- 
207 

see also Communication equip¬ 
ment, design considerations 
portable radio equipment, 193 
public-address systems, 193 


radio link, 197-207 
shallow-water diving, 194-196 
sound powered telephone, 188- 
193 

voice tubes in submarines, 196 
Voice-operated relays, 146 
Volume indicator, speech, 60-63 
Vowel triangle, 60, 65 
VU meter (volume indicator), 60 

Wasmansdorff noise limiter-, 198 
Weight law of sound, 20 
Western Electric 

640-A condenser microphone, 92, 
142, 148, 164 

640-AA condenser microphone, 
163, 164 

751-B loudspeaker, 219 
MC-253 microphone, 92, 239, 250 
Western Union vocabulary list, 82 
Wind tunnel, airspeed studies, 241- 
248 

determination of velocity distri¬ 
bution, 253-254 
high-speed tunnel, 243-248 
low airspeed, 241-243 
propagation of acoustic shock 
waves, 247 

striation apparatus, 243-245 
striation photographs, 247 
test results, 245-248 
W-pass circuit, radio receiver, 201- 
202 

ZA 13935 British throat micro¬ 
phone, 132 

ZA/CAN 5155 Canadian micro¬ 
phone, 132 

Zirconium oxide in incandescent 
light source, 269 


DECLASSIFIED 
By authority Secretary of 

OCT 101960 

Defense memo 2 August 1960 


library of congress 






1130 



^.CLASSIF IES- 

By authority Secretary of 

OCT 101960 

Defense memo^A ugu9t i960 

„ h «ary OF congress 



thK .TEM CONTAINS Sm ILLUSTRATIONS WHICH 

BE OU^SATlSFACTOni V LMITHET 

THE EUCTRWT flC OR THE PHOTOS i AT 1C PROCtSo. 





































































































































































































































































































































































































































